The first thing I would try is cleaning out the heatsink and fan assembly used to cool the CPU and GPU. If that is too clogged with dust and debris, no amount of external cooling and opening parts of the system up is going to help with keeping it cool enough to operate.
If the system is servicable enough, you could potentially open the case up to expose the cooling system and clean it directly. If not (meaning cleaning that you would basically have to completely disassemble the unit for that purpose), then the only "easy" thing to do is try a can of compressed air blown into the vents to try and dislodge dust and debris. Use the straw that most of those cans come with and get it up against the vents on the bottom, aiming as best you can so the air blows the dust OUT of the computer.
If the machine doesn't have vent holes on the bottom, only on the sides, then all you can do is blow in with the air to clear it out. That's not the best way, since it's basically blowing the dust into the fan and into the computer interior, but if it's all you have it's all you have.
I don't like recommending that, usually, as I've heard SOME stories at times of it causing damage, but it DOES work and I personally have yet to encounter any fatal issues because of it. It's also the EASIEST, if not wholly effective, method to start with. It's the easiest way to clean out the vents on my DV9700, which otherwise requires a complete teardown to clean.
If it's NOT dust and junk clogging the heatsink and fan, then you may be facing some level of hardware failure. In that event, I would check the Event Viewer in Control Panel/Administrative Tools for error events around the times of your crashes to see if Windows has reported any errors and what they might be.
Hope this helps some!
O Shadow Fox O