Q. In Norfolk I used a regular Internet service provider and downloaded RealPlayer. When I moved here I signed up for America Online, and now my RealPlayer doesn't work.

A. Homeowners know that you can mix different paints and get new colors; they also know that once you've added a color, you can't pull it out. You have to start again from scratch. We have a similar situation with Internet service providers.

Let's say you start off with an ISP and decide to use Netscape as your browser. Browsers these days have a modular design allowing you to add special small programs called "plug-ins." You download them, and your browser installs them in certain places. Real Networks made a name for itself years ago with its RealAudio plug-in. Today's featured product is called RealPlayer Plus G2.

You would think that if you changed ISPs, your old plug-ins would still be there. They are; it's just that when you download the software for your new ISP, your computer forgets where to find them.

It is possible to do some fancy reconfiguring to fix that. The easier thing to do, though, is to start from scratch. Install the new ISP software and browser, and then go back to the Web and get the plug-ins.

How can I get rid of a virus when access to the infected file is denied?

Mom always told you not to take candy from a stranger; on the Internet you shouldn't even open attachments from strangers. Our forlorn reader has accidentally placed a "Trojan horse" virus called APStrojan.gen on his computer. This particular beauty tries to steal a user's password and e-mail it to someone else.

It enters a computer as an attachment to an e-mail message. This subject line may be some come-on about getting a free computer. When you open the attachment, you inadvertently install a small virus on your computer called a "terminate and stay resident" program, or TSR.

Our reader's anti-virus program has identified the problem. But it can't clean the virus because one of its tricks is to install itself as protected files. For good reasons, computers designate certain files as "read-only" or "system" so that well-meaning users don't accidentally delete them. Virus writers sometimes take advantage of this protection.

But since the virus scanner can give the file's name, you can take it out manually, then reinstall the file from an uninfected source. The best bet is to boot to a DOS prompt and delete the file, though you may have to use special commands that can be found in your manual. If you're brave and patient, you can reboot and go to your computer's registry, a sort of clearinghouse about its software, and try to edit references to the Trojan from the registry. You get there by clicking on "start" and then "run" and typing c:\windows\regedit.

I changed a file extension to "wav," but my computer says it is not a wav file.

I can put a Mercedes-Benz hood ornament on my 1987 Taurus, but that sure won't impress my neighbors. The same applies to the naming convention we use for personal computer files.

It's based on good old DOS, the pre-Windows operating system whose influence we still feel in Windows. In DOS, the characters to the left of the period are called the file name, and the characters to the right are called the extension.

Extensions for sound files include "wav" and "midi." Wav files use a sound "waveform." The format was developed by IBM and Microsoft and has become the de facto standard in the Windows world we live in. Midi stands for "musical instrument digital interface."

But changing the extension to "wav" doesn't make a file a sound file. You need special software to create a wav file. Sonic Foundry has a product called Sound Forge XP 5.0 that sells for $44.99.

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. 20071-5302 or via e-mail at jgilroy@iteminc.com.