The Warning Signs of a PC Infected with Malware

Many readers asked for more information on symptoms they should look for if they fear for their machines' security.
Subscriber Leslie Kight asks the following question:

• "Great article. I'm curious, though: what makes Woody suspect his XP machine is infected by Mebroot? What symptoms did he see to raise that question?"

Here's Woody's reply:

• "I kept getting weird virus warnings from AVG — viruses would appear, I would remove them, then they would reappear in different locations, or entirely different viruses would show up. AVG reported that the MBR [Master Boot Record] was being changed every time I rebooted, even when I did nothing.
  "I did a deep scan — first with AVG, then with NOD32 — to remove all the reported malware, but the viruses kept reappearing. Antirootkit scans turned up nothing. Then I couldn't connect to F-Secure's Web site, so I pulled the plug.
  "As I said in the article, I have no idea at all if it was Mebroot. But I couldn't find any reports of similar collections of problems and decided to err on the safe side.
  "Periodically reinstalling Windows is something I recommend anyway: once a year is ideal, in my experience. I'm happy to report that I've reinstalled XP Pro (SP3, of course), reactivated [Windows], and brought back the data files; everything appears to be working just fine. The machine's snappier than ever."