Computer turns on and off by itself

I have a compaq that i have had for almost 2 years,, i ran spyware and virus scan on a regular basis. One day my computer began to turn itself on and off, i have used the built in full system recovery, cleaned all my fans, checked my connection points, and disconnected that computer from the internet yet it continues to boot and shut down on its own.. any help would be greatly appreciated.

Go into system CMOS (usually DEL or F2 or F10 keypress on system startup)

Check any wake up commands, and disable them.
Check Power Management settings as well.
If you are unable to find these settings, just load bios defaults (or optimized defaults in cmos)

Failing this, check any plugged in PCI device (ie 56K Modem)
Or possibly replace the power supply