Q. On the radio you talked about a memory leak. What's that?

A. A memory leak usually refers to a problem in an operating system or the physical memory of a machine.

Years ago, when memory was very expensive, computer code was written to be as short as possible. People would use words such as "elegant" to describe a well-written, efficient program. When memory and hard drives got cheap, code got sloppier and sloppier. By the time Windows 3.1 became popular, some programs would not let go of memory they were using, even after you closed them out.

This was commonly called a memory leak. Your system would not recognize all the memory that it in fact had available, and you'd get an error message in error. The work-around solution: Turn the system off and restart.

Another kind of leak involves bad physical memory. Let's say you have a memory card configured with four 1-megabyte memory modules. You notice that you occasionally get errors with a graphics program. It is quite possible that the final memory module has failed, but you only encounter the issue when you use a graphic program that uses all of the available memory.

To troubleshoot this, swap out the modules--put number four where number one is and see if the problem pops up sooner. If it does, you know you have a bad memory module.

Q. Who is Alexa?

A. Not who, but what. Alexa (available at www.alexa.com) is a free add-on to your Web browser that helps your surfing. My favorite phrase for Alexa is "browser companion." Named after the great library of ancient times in Alexandria, Egypt, Alexa was developed by a San Francisco company called Alexa Internet that was recently purchased by Amazon.com.

Founder Brewster Kahle was tired of getting lost in his Web searches and getting error messages, so he developed a product that helps out. For example, if you go to a site and get a "404" error, meaning the page has been removed from the Web, Alexa can take you to a compendium of old Web sites to see what the old one was like.

Information is king on the Internet, and Alexa offers an insider's page that lists top daily Web pages. When Alexa is installed on your browser, you see a small tool bar at the bottom of your screen. When you go to a Web site, you can see sites that other visitors have liked. It can also show you how popular the site is, as well as how many sites it is linked to. With that much information, you can spend a day at your competitor's site and learn a lot.

Q. I heard there is e-mail software that will send you an e-mail with a return receipt.

A. There is, and it comes with varying levels of usefulness and reliability.

For example, if you use Eudora Pro version 4.0, you will see a small button on the screen marked "RR"--this stands for return receipt. Assuming Eudora Pro is in use there too, the recipient gets a note that says "the sender has requested notification that you have seen this." They are offered several boxes to select, with options to notify the sender at that time, later or never.

If the recipient chooses, he can make the program send a notification. This is a nice system for ladies and gentlemen. But if you send a nasty note to a deadbeat dad, he will never acknowledge receipt.

There are several commercial systems now that take things a step further, allowing people who don't use the same software to trade mail and get verification. They include Certifiedmail.com and UPS Document Exchange. Basically, you post your message on a secure Web site, the recipient is notified to retrieve it, and you get a confirmation when the message is received.

All of these systems raise the following question: What if someone else turns on my machine at home and downloads my e-mail? The person might even be polite and acknowledge receipt of my Eudora mail. But that doesn't mean I got it.

John Gilroy of Item Inc. is heard on WAMU-FM radio's "The Computer Guys" at 1 p.m. on the first Tuesday of the month. Send your questions to him in care of The Washington Post, 1150 15th St. NW, Washington, D.C. 20071, or via e-mail at jgilroy@iteminc.com.