Amy DiAngelus's laptop computer erupts like a noisy telephone whenever a loved one enters cyberspace, alerting her with an assortment of human squeals and musical flourishes.

"Hooo-haw" means her mother in Florida has logged onto the Internet and is eager to chat -- trade live messages at the keyboard.

A famous salty line from a movie means DiAngelus's husband, Dan, a consultant who travels up and down the East Coast, has signed on somewhere -- it doesn't matter where. A tiny box beside his name on the screen signals his availability to talk; it's the electronic equivalent of eye contact.

"I can message him and tell him that our 2-year-old just said, `I miss Daddy,' " said DiAngelus, 24. "It's better than calling him at the office all the time and disturbing him."

Free software called ICQ -- "I seek you" -- is behind this magic in the DiAngelus family. They are among 35 million people who have downloaded copies of the program that America Online Inc. bought for about $300 million last year from the Israeli company that developed it.

Loaded with clever features, graphics and sounds, ICQ allows people to send private notes to friends anywhere on the Internet and detect their presence online. It lets them find people with similar interests, as well as maintain to-do lists and calendars and perform other personal tasks. It's not entirely easy to use -- setup can be difficult -- but more than 70,000 more people a day are finding it's worth the trouble.

Much as electronic mail created new patterns of communication in the early 1990s, today ICQ and other forms of "instant messaging" are changing the way millions of people stay in touch and manage their online lives.

A few analysts think that, over time, these instant-messaging programs could even challenge the market dominance of Microsoft Corp. and its Windows technology. The more features are loaded into ICQ, the more software other companies are likely to write for it, and the less time people might need to spend with Microsoft's desktop software. Microsoft has its own instant-messaging system, due to be released within three to four months, but the company's efforts have been plagued by more than a year of delays.

ICQ's power stems from its ability to make cyberspace feel more social. "When you log onto the Internet you are surrounded with the group of people you know, and you share the experience -- it is very exciting," said Sefi Vigiser, 26, one of four Israeli programmers who founded ICQ.

DiAngelus, who is raising two preschoolers at home, typifies the heavy ICQ user. When her husband had a lengthy consulting contract in Philadelphia, she used ICQ all day long to give him an account of their son's potty training and their daughter's activities.

And because her mother loved the "hooo-haw" sound that Al Pacino made in the movie "Scent of a Woman," DiAngelus downloaded a movie sound clip and assigned it to her mother's ICQ name so her own computer would make that sound whenever Mom logged on. "I missed her greatly, and it made me smile every time I heard it," said DiAngelus, who moved back to Florida from Columbia a few weeks ago.

To widen their messaging circles, ICQ users sign up friends via e-mail, creating an unpaid sales force that has given ICQ one of the largest databases of user names on the Internet.

ICQ could also become a key tool for Internet commerce. Instant messaging is creating new and strategically important tracts of real estate online. That's because the software claims a corner of the user's computer screen and stays there, unlike Yahoo, Amazon.com and other World Wide Web sites. The corner becomes, in effect, a billboard for whichever company can afford to put ads on it.

AOL also has its own chat software called AOL Instant Messenger. It does far fewer things but is simpler to use than ICQ, works both inside AOL's proprietary service and outside and is freely available even to folks who do not subscribe to AOL.

There are other messengers from Yahoo Inc., Walt Disney Co. and a company called Tribal Voice Inc. And while Microsoft is hard at work trying to integrate a new instant messenger into its Hotmail e-mail service, AOL is preparing a free e-mail service for ICQ.

AOL's two programs have spread so fast that the Dulles-based online giant has a virtual monopoly in messaging, with at least three-quarters of the market. In April, ICQ drew more usage time than any site on the Web, according to measurement firm Media Metrix Inc.

More than 14 million people actively use ICQ each month, sending 330 million messages on a typical day. It rivals traffic on AOL Instant Messenger, which an estimated 19 million people used last month to send an average of 432 million messages a day.

ICQ has been adding features since it was first put up on the Web in 1996, in an effort to make it more like an operating system than a chat tool and to give people reasons to use it longer. Internet telephone service has been added, as has a search engine, a pop-up calendar that can work like an alarm clock, and directories to help ICQ users connect with others who share their interests.

The goal, AOL executives said, is to make ICQ one of the grand central stations of cyberspace, helping people maintain a single online persona through the unique number ICQ assigns to them. The number injects spontaneity that had been missing from the Internet by allowing people to "dial" each other and establish telephone-like connections. The number eventually could make people accessible to friends -- and favored merchants -- from many devices: cellular telephones, pagers, televisions.

People might use ICQ to order movie tickets, make hair appointments, shop at their favorite stores -- even allow certain merchants to make direct pitches.

"Just as you've got your friends online, you'll have your merchants online," said Ted Leonsis, president of AOL's Interactive Properties Group, pointing to the box displaying his ICQ contacts. "Your insurance man, your lawyer -- they should all be here."

Leonsis said ICQ users will be able to "invite" merchants onto their contact list in the next version of the ICQ software, due this year. AOL's challenge is to sign up companies that can take advantage of the instant notification function and appeal to its young audience.

Leonsis said ICQ can't reach its potential without businesses. Noting that nearly 15 million people actively use ICQ, Leonsis said, "It's like we have given those people telephones but no businesses have telephones yet."

Some ICQ users remain skeptical, worried that AOL's commercial plan will spoil their social experience. "I hate the idea, but nothing in this world is free," said software engineer John Doktor, whose attitude typified most ICQ users interviewed.

ICQ's audience has been dominated so far by young people, mostly outside the United States. They include people like Doktor, 25, who began using it after his employer transferred him to England from Denmark in January.

With ICQ, Doktor was able to see his friends as tiny buttons on his computer screen and to watch his girlfriend's name turn to blue from red whenever she signed on back home in Denmark. "Those little blue names make me feel in touch with people," he said. "If you need to speak to somebody, you click on their name and there they are."

ICQ is gaining ground for communications within businesses, too. Eric Anderson, vice president of a technology firm in Alexandria, said he stays in touch with colleagues across the country via ICQ because it is quicker than the phone. "It's just like leaving a quick voice mail," he said. "I think it will be a huge productivity tool one day in the not-so-distant future."

ICQ's history in many ways mirrors that of Netscape Communications Corp., the company that gave away its Web-browsing software for free and then had to figure out how to make money from its rapidly growing audience. Both were founded by programmers in their twenties who wrote software code in an effort to make the Internet easier and more lively for ordinary folks.

Both attracted large audiences ultimately coveted by AOL -- the company that has gone further than any other in turning new-media audiences into advertising dollars.

Vigiser and three pals dreamed up ICQ in 1995 after finishing their required three years in the Israeli army. They coined the program's name by adding the more personal "I" to the "CQ" that ham radio operators had long called out over the airwaves.

They named their company Mirabilis Ltd. when one of their fathers looked up the Latin word for "marvelous" and found it was "mirabilis." The word also was the name of a flower, so the youths adopted a daisy-like logo as one of the program's quirky tools for managing social contacts online.

People can choose from eight flower icons to signal different levels of openness to communication.

An open green flower, for example, means someone is available to communicate, while a "DND" stamp on the flower means "do not disturb." An eye inside the flower means the person has chosen to be "invisible," which means other ICQ users are unable to tell whether the person is online.

Letha Young, an 18-year-old college student in Fort Worth, said she considers ICQ a lifeline to her family in Japan. During her first year of college, which just concluded, she could not afford to call home regularly, so she put more than a dozen relatives on her ICQ contact list and signed on daily to see whose name was blue. "I don't know where I would be without it," she said.

Letha's mother took the trouble to learn ICQ to communicate with her children. Often impatient, Young signals her desire to chat by telephoning her mother in Japan, letting the phone ring once, and then hanging up. "We do that with my sister, too. "

Often, the three women open a private chat window that fills their screens and allows them to type longer notes that appear instantly on all three computers. They enliven their chat with colors, acronyms such as ROFL -- "rolling on the floor laughing" -- and "emoticons," short strings of characters that symbolize a smile, frown or other expression. "It's almost as if you can hear their voice as they type, especially when they're making smart remarks," Young said. One night her sister announced that she was going to buy a suit of armor for her dormitory room. "Whatever! You are weird," Young and her mother typed simultaneously.

ICQ offers a feature called "random chat" that automatically connects people with strangers, but many people say they don't use it. "I have one Net friend that met a man through random chat, and he turned out to be a psychopath," said Elizaveta, a 27-year-old restaurant and bar manager in Moscow who declined to give her last name.

She said ICQ's constant interruptions can be annoying. "When I'm involved in a chat or just waking up to read My Yahoo news, I don't want to be hassled, so I'll put up the "DND" sign. Problem is, most people are not very respectful of netiquette and beep me anyway. If it's too much, I'll go invisible."

ICQ users can't communicate directly with the AOL's InstantMessenger or any other such program. AOL could make the two programs compatible but has shown no interest in doing so.

Leonsis maintained that it is because the ICQ and AOL audiences are different. Some industry analysts, however, believe AOL doesn't want to take any steps toward open-messaging standards that might make it easier for competitors to communicate directly with AOL's two messengers.

Jakob Nielsen, a Web engineer who has studied software design for more than two decades, said different systems ought to be able to talk to each other. Nielsen said the situation reminds him of the early days of telephones and e-mail, when competing systems sprang up that could not talk to each other. AOL's view "is like saying I have a black telephone and have no need to talk to anyone with a pink telephone," Nielsen said.

"Ultimately, it will be universality that makes this medium succeed."

CAPTION: Amy DiAngelus uses the ICQ instant messaging service to check in often with her husband, Dan, a consultant who travels a lot. Their two children, Braeden, left, and Cailyn look on.

CAPTION: The Message of ICQ: ICQ (I Seek You), which looks like this on a personal computer, and other forms of "instant messaging" are changing the way people stay in touch.