If it's still running, means it's not broken yet. Search on youtube how to dissemble a laptop (http://www.youtube.com/watch?v=Bwr3vxSErs0).
Dust it, reapply thermal compound if you can. Put it back and see if it's running at normal speed again. If not, buy a new one (about $20-30) and install it. It should fix the problem.
If you're too lazy to do it, bring it to the store. Of course it would cost more.
If you don't want to do that neither, buy a external cooling pad for laptop, a good one. It works (at least to my brother), but not as good as internal one.