I know its a pain, but the only way you are going to really find out probably is to reinstall windows, maybe take it in for a cheap cleaning if you dont want to do it yourself, and see if the problem persists. It sounds like a cooling issue, maybe over clocking but I doubt it unless you are playing games, maybe the CPU heat sink is clogged or loose, cooling fan may not be working, but it definitely sounds like a CPU issue, sort of a thermal shutdown.
It could also be antivirus software issue, and it maybe just time to get a new computer too but I am guessing a cooling issue like some have already said on here.
Are you getting a chksk on reboot?
If you dont want to reinstall windows I would download motherboard monitor or speedfan or coretemp and see whats happening with your temperature, if you have an apple you dont need all this crap but they do hae one freeware tool out there but you really dont need it, its built in but this problem sounds like its happening on a pc.
If your computer is old, buy a powerbook or macbook , its worth the extra money.
also like nina said dont surf for porn, in fact the gals on here maybe be able to tell you the best virus free sites, they sound like a wild bunch.
cheers