Faulty power supply.
incorrect CMOS settings
Wake on lan (if network card is present)
A windows shedule may be enabled.
If XP you might be going into hybernate or sleep rather than shutdown

shall I go on..

I would look at resetting your CMOS for starters.
Then check if you have a sheduling agent that is starting the PC.
Disable Wake on lan in bios as well.
Remove your network cable (if you have one.. you might be getting a spike from a Lan\Router\hub !

Q. do you maintain your PC hardware yourself ?? if you do, have you upgraded.. changed the PSU? is it giving out too much juice ?

Good luck