It’s a few years old 2.15 GHz Intel Core Due, 2 GB 667 MHz DDR2 SDRAM using Mac OSX version 10.6.8. It seems to overheat whenever I’m trying to do something interesting.
My daughter’s macbook did that when the fan died.
Dead fan or vents clogged with dust.
Macs can have problems? 
I ruined my first laptop by not cleaning the dust out of it. Get some canned air and spray into the vents against the normal flow of air (while the laptop is off). I recommend doing this once a year, twice a year if you have cats.
Im not a Mac guy so Im not familiar with the fan problem.
You can also consider a laptop cooling pad like this one, but that’s secondary to clearing out the dust and making sure the fans are still blowing.
MacBook Pros are notorious for thigh-burning temps. Blow out the dust bunnies and make sure you don’t put it on a pillow or blanket.
Don’t know about MAC’s, but I had to replace the fan on my PC. Cost a whopping $24. Very easy to replace, but seemed daunting at the time–just buy a jeweler’s screwdriver set. On PC’s there are free app’s that you can download to monitor CPU temp. I would assume that MAC’s have similar app’s available–find one and determine what the CPU temp is when idling and when crunching at 100% CPU. If it is getting over 70* [C] you have a fan/cooling issue. New fan dropped my PC from 80* to 50* when cranking at 100%.
Was it a desktop?
Fan replacement on a MacBook laptop might not be so easy:
MacBook Pro 15" Core Duo Model A1150 Left Fan Replacement - iFixit Repair Guide (just an example, not necessarily the right instructions for the OP’s particular model)
DO NOT try to take the screws out of your MacBook’s case unless you have the correct screwdriver size (a 00, I believe). I tried to do mine with a screwdriver that was the next size up, and almost all of the screws came out with no problem. But I wound up stripping the last one, and ultimately had to take it to the Apple store so they could get it out for me. (Which they did, and then I replaced the battery with no further issues, but still. Get the right size screwdriver.)
This might be totally obvious to anyone with half a clue, but I wish someone had told me before I sat down to do what was supposed to be a really quick and easy battery replacement.
Correction: I was upgrading the RAM, not replacing the battery. This is what I get for late-night posting.
No, laptop. Old Dell Latitude D610.
Install Istat Pro into Widgets and see what the temps are actually. Then install Download smcFanControl for Mac | MacUpdate and cool it off.
Then do all your diagnostics with Mackeeper or equivalent and blow out Safari and Firefox caches with a reset. Check Activity Monitor and find and kill any parasitic programs like HP Print Monitor that will automatically grow until the machine stops.
Get yourself a lap desk, first of all. Then, I recommend a non-powered cooling pad that just raises the bottom of the laptop up so that air can flow all around it. I’ve used and loved this one and this one from Targus. The pads lift the laptop to a comfortable typing angle and even allow you to rotate the laptop easily like a Lazy Susan, so you can pretend you’re Captain Picard.
I think your machine is just new enough so you shouldn’t have this issue, but the earlier Macs have a situation where the heatsink paste loses it’s hold or something. These get really hot just above the #1 and #2 keys, right by the hinge. If it’s hotter there than anywhere else, do some Googling.