I have the same problem with my HP especially while gaming or playing Hi-def movies.
intel core2 2.5GHz
4G RAM - 512MB NVIDIA
This works for me most of the time. I keep my laptop elevated from the resting surface (such as a table, etc) by about 2-3 inches by supporting the laptop on a box or book at two ends (Make sure the RAM and fan areas are unblocked by whatever ur resting the laptop on). Also I keep a small table fan on next to the laptop to make sure that there is cool air being blown across the bottom surface of the laptop.
I know this is very abstract way to deal with the issue but it works for me...usually a game/app that would overheat my laptop and cause it to shut in 20 mins or so, no I can run it for upto 3 hours continuous easily with this method.