Computer switches itself off?

[KAZ]

My Computer has recently started shutting itself down for no appararent reason? It happens just as its booting up or sometimes I manage to open a program or webpage and then after a couple of minutes everything just switches off. I can turn it back on again and it reboots fine, no error message . It does this up to 3-4 times in a row then seems to work fine after that....Any suggestions????? I am using Windows XP Home

[druiaghtagh]

Last time it happened to me it wasa virus called Blaster, may be another more up to date variant