Start with what nightfly said. shut the computer down, and unplug it, and get a vaccuum cleaner to clean around any of the fan ports. clean around the inside, but be careful not to hit anything and knock it loose. do not touch any components on the inside with your hand unless you are touching the inside of the case at the same time. Touching the inside of the case will discharge your static, and keep you from building any static while you are touching it. the more dust and gunk you can remove from the inside, the more air can flow to help keep it cool. if you can get to the processor heatsink, you can carefully use a Qtip to clean betweent the fins if they are spread enough. heat sink will be a square usually, block of alluminum with a lot of fins on it. Don't remove it cause there is a heat paste under it, and putting it back on properly is sometime a real chore.
this will only help if it is a temperature problem. but if the comp has any age on it, and this has never been done, then its a good place to start. and makes for good practice anyway. dust is your enemy on electronics period.
to use the speccy program, just download it from http://download.cnet...4-75181811.html install this program, and let your computer just set for a few minutes, and get a reading by just double clicking on speccy. the main wiondow that opens by default will tell you temps of processor, graphics card, and hard drives.
let us know what the temps look like. and then go play a round or 2 of your game, and exit it, and immediately run speccy again to see the difference of temps.
then we'll go from there .