My computer has been shitting itself down whenever I'm doing something processor intensive like rendering something. It also does it over night, O assume when it's scheduled to do a virus scan.

I've never had this problem before. I went a computer place and the guy said it sounds like it could be the power supply but he didn't exactly seem like he knew what he was talking about.

I thought maybe an overheating issue but I opened the case and stuck a fan in there and the temp dropped by almost 10º so i don't think that could be it.

Any ideas?

That title was supposed to say "Again in need of the computer whiz kids"

My first guess would have been overheating, too.  Good call.  Although it's still possible the power supply is overheating.  Did you give everything a nice clean-out with an air canister?  Remember to hold all fan blades when cleaning them off…spinning them with compressed air could foul the bearings.

My second guess would be RAM.  Have you tried letting it go overnight with the virus scanner turned off?  My guess would be that if processor intensive computations are taking it down, it might be because you're hitting a bad sector in the RAM and the comp is failing.

And how exactly is it shitting itself.  Does it turn completely off?  Reboot?  Blue screen?

Usually whenever I get weird behavior like that I start stripping the computer down to bare bones.  No CD-ROM drives, minimal HD and RAM, no sound card or network card.  If it works, I start adding components one by one.  In your case it might be a little difficult unless you can find out how to replicate the failure on a consistent basis. 

Unfortunately I'm having problems with my comp as well.  The damn thing won't even turn on.  I'm waiting on a new motherboard AND power supply from Newegg.

It's basically just powers itself down down. Doesn't reboot, no blue screen or anything like that.

Hmmm…does sound like the power supply.  The processor intensive activities could be drawing more load from the power supply.  Or even the processor fan could kick the PS into high gear.  I would also check the motherboard for any bloated looking capacitors.  Is it a Dell?  Certain Dells tend to have capacitor problems.