Andy....
Microsoft have a page about the Sasser Worm here http://www.microsoft.com/security/incident/sasser.asp
Basically it can infect your PC by getting in via unprotected internet connections. It causes computers to shut down. If you're using your PC and a window pops up to tell you that windows is about to shutdown (you get a count down to zero and then windows closes) you're probably infected.
Pete Little
May 2004