Since you have a store bought computer(bleh), then there should only be one spot where you can put a fan and that ist he outlet.
Open up your case, and look for those holes next to where you plug things into your motherboard(keybaord etc), The wholes are formed in a shape where you can just bolt on a fan, in this case you will want the fan to suck out the hot air.
How exactly do you know your computer is overheating? Stock systems should not overheat, check for dust that might have clogged up.
Something stated 3 posts up would be a good idea if your system still overheats, if all else fails, move your computer to a new case.
G. Skill 4GB(2x2GB) DDR800
eVGA 8800GTS 320MB
super pi: 1m=20s