The CPU should auto throttle (close part of itself off) to try and cool down and prevent damage when running too hot. All mobile CPU's will do this and laptops with Desktop CPU's (P4's) will also do this to try and save battery life (even if temps are ok).

Get a screw driver out and see if you can get to the fan, give it a good clean and clean off the mat of hair/dust/finger nails that will have stuck itself to the heat sink.

If you cant get to the fan and sink then get a can of compressed air, try maplin, and give it a good blast through the fan grates at the back.

The fan could be dead but i'd try giving it a good clean out first.

Never use a laptop on your lap or on a soft surface, like carpet or the sofa as it tends to block the airflow.

hth.
JAmes.