Does This Look Infected?

Today, after turning on the PC, I get some message on the screen telling me that something went wrong and the computer will shut itself off in one minute. I restarted the comp, and everything was fine after that. Later, online (using the Mac) and I see an adversitement displaying the same message that I had seen earlier on the PC. I click on the ad and the site it takes me to says that the message I had received was caused by a virus! I couldn't believe it. All virus definitions had been updated throughout the week, I scanned all email attachments I had received, and we're behind a router. However, the night before I did use WinMX unsucessfully, leaving the program running for at least 6 hours. So maybe doing that let the bug in. Just to be sure, I went to Symantec and did an online virus scan. The results were clean, but that didn't expain why the message showed up. Maybe if I just leave it alone, the problem will go away by itself. *Crosses fingers*