Blimey... in that case that could well be your problem then.
First of all, disconnect your pc from the mains. Then open the side panel, (usually held on by a couple of screws), to reveal the insides.
Once inside, be careful what you touch with your hands, as you can pass on static electricity through them. Just make sure you keep touching the inside wall of the tower... this will help discharge any static.
I'd avoid touching any cards or electrical circuits too as it's easy to do damage.
Don't attempt to move anything, or disconnect anything if you're not familiar with the inside of a pc. Just try and remove any dust, and crap that has built up over the months/years.
Apart from that... you can't go too far wrong.
Here's a guide i found on the net that might explain it better than i can. Don't worry about all the tools they say you need.... you really don't need half of them.
Hope this helps.... and cures your problem!
http://www.bleepingcomputer.com/tutorials/tutorial118.html