Those very irritating people who design viruses and malware have come up with a horrendous new one now. Alongside the Antivirus 2008 / 2009 nasty (which keeps telling you that you have loads of infections so please give them your bank account / credit card details so they can empty it for you) they have now added a BSOD (blue screen of death) screensaver whcih gives the impression that your PC is crashing and restarting regularly. The system I saw also had the desktop wallpaper changed so it showed a warning screen and disabled the relevant display property settings so not very easy to get back to normal.
Avoiding this nasty is tricky but first make sure you have up-to-date anti-virus software such as Free AVG version 8.0 along with Adaware 2008 and Spybot 1.6 to keep spyware at bay. Remember to update and run Adaware and Spybot on a regular basis and consider using an alternative web browser such as Mozilla Firefox.