Hum, you might want to really look into your problem more I think thats a hint that something may give way down the road from pro-long use
, personally If it where the memory you would be having much more difficulties booting or logging windows. (more errors on the software end)
If you still experience the problem in a short amount of time (be it the resets) and your software that you run be it windows or COD is working without crashing or errors, consider your main diagnostics for this problem, because " Computer wont boot up?" could mean a whole lot.
1. (This is the first step i would do) Remove all Power connections from the motherboard be it a battery or the ATX power cable and the 12V+ Rail, all located on the motherboard. And keep them removed for about 3 hours. Yes 3 Hrs, being because it needs time to dis-charge the capacitors both in the power supply and the main-board.
If the PC then boots from trying 1. but again starts to act up, a whole lot of diagnostics could make a list... But overall, the motherboard is getting OLD, and is probably bound for a replacement.