reboot problem

HI,my pc keeps rebooting for no apparent reason.no warning message and when i my virus ,spyware and windows care programs they say all is ok.is there a program out there that can better diagnose these problems or does anyone know why this is happening.my system is a abit fatality 775 board,p3.2 520,250gb harddrive and gigabyte x800 graphics card.all have the latest drivers.thanks in advance
a10538:confused

 

Either a heat issue (internally) or a boot sector virus. Go to:
http://forums.majorgeeks.com/forumdisplay.php?f=35 and follow the steps for malware removal.

Did you install any software prior to this happening?
Did you install any hardware prior to this happening?
Did this happen all of a sudden?
How old is your machine? (roughly).