First, make sure you actually have a virus. If you believe you have a virus because you received an email from someone telling you that they passed one along to you and they're sorry, you probably don't have one. There are many virus hoaxes that continually circulate around the Internet, causing people to worry unnecessarily and even delete files that are supposed to be on their computer. Some good examples:

jdbgmgr.exe
sulfnbak.exe

If you aren't sure whether something is a hoax, try looking it up here:
About.com's Virus Encyclopedia

That site also includes great information about preventing viruses, repairing damage from a virus, and breaking news about new viruses.

Now, if you still believe you have a virus, you may actually have files at risk, so limit your actions on your computer until you can get it checked out. Many viruses aren't really damaging, but some can be, so either check your computer with anti-virus software or have someone at a computer repair center check it.

We strongly recommend that computers used to access the Internet and exchange files have anti-virus software installed on them. Also, do not accept email attachments from persons you don't know, and be wary of them from persons you do know if you weren't expecting them (or even if you were, just to be safe). Keep in mind that just reading a standard email message does not carry danger, but attachments in emails can be infected.