From your post i guess you are using Windows Xp and intel processor , However this will work with vista & 7 also.
1>Format your PC
90% chance it will be OK ,if it doesnt help then
2>Check your north bridge-if it is hot then put some thermal paste and add an extra fan on your north bridge.
4% chance it will be OK ,if it doesnt help then
3>Check your heat shrink on top of your processor-if it is hot apply thermal paste and add some cooling device.
2% chance it will be OK ,if it doesnt help then
4>Check your GPU's Fan (if you have any) ,if it is hot then its time to replace your card and add 2 to 3 fan in the cabinet.2 outflow 1 in flow direction
2% chance it will be OK ,if it doesnt help then
5>Overhaul your entire pc + clean your entire pc + give it a break for 2-4 days and rearrange it
If problem persists.....1% chance leave it on god.