Viruses seldom reveal whose computer is actually sending them. They commonly fake the 'From' address of the e-mail, often using an address found on the infected PC's hard drive. The virus probably came from an infected machine belonging to someone who knows both of you; it sent itself to every address on the hard drive, randomly picking yours as the "sender."

On the other hand, be very suspicious if your PC uploads files over the Internet without your approval. Much malware today sends info from your PC, either to spy on you or to use your PC to send spam or a virus. Make sure your firewall is set to stop and report on all outgoing activity you haven't explicitly approved. Windows XP's firewall doesn't provide this functionality, so if you don't already own a security suite or stand-alone firewall that can handle the job, I recommend that you get Zone Labs' ZoneAlarm or Sygate Personal Firewall, either of which is free for personal use.

You might discover hints of an infection in the System Configuration Utility, also known as Msconfig. To open it, click Start, Run, type msconfig, and then press <Enter>. Click the Startup tab. In the list of programs that load automatically when Windows starts, look for one whose name resembles an eye chart; many malicious programs rename their files with random character strings. Others, though, mimic the names of real system components.

If programs like Msconfig, the Windows Registry Editor, and your antivirus program don't load, your PC is almost certainly infected (although these programs sometimes act up for reasons other than a viral infection). For details, see my August 2005 column, "Prevent Viruses From Disabling Your Protection."

Free online virus scanners can help you find the culprit if your regular antivirus program is compromised. I recommend Bitdefender.com (see Figure 1), Kaspersky Lab, and Trend Micro.