If you're comfortable about opening up the pc and rummaging around inside, remove the memory cards and put them back in (obviously do all this while it's switched off and unplugged!), remove any additional cards you have in your pc (graphics card, tc card etc) and reboot. If it boots, put the cards back in one at a time, rebooting each time. If it works the first time but fails when you put a card in you'll know it's that card at fault. It could even be the memory but I dunno if you can boot up with no memory installed, you could try - you'd probably get beeps telling you there's no memory maybe?
It also gives you an opportunity to clean out any dust that might have collected in there!