Computer restarting randomly (yet another one)

[Loopy]

Ok, so here is the whole story.
At first my PSU died on my old computer, but since i didn't realize that, i thought it was the motherboard, so i bought a new one.
Then obviously with the new mb comes a new proc and new video card and the new PSU. Then i started getting weird restarts, and i couldn't even install windows (it would just restart by itself on random occasion). Then i started thinking it was the memory, so i bought that also, but alas it did not help.

So now i have a brand new computer:

ABIT SG-80DC
Pentium D 3.2GHz + a fairly strong and big fan
1GB DDR 3200
GeForce 6800 256mb
80 + 40 gig hard drives (freshly formatted)

.. that doesn't wanna work.

Eventually i managed to install windows, and the restarting appeared to have stopped, but as soon as i started playing games, or doing anything that needs more power from the computer, it would restart.

Now, i've been looking EVERYWHERE for a solution. Keep in mind that I:
Have no dust in the case.
Have checked all the cable connections
Have reseated everything that can be reseated on the motherboard.
Have tested: 2 different video card, 3 types of memory, 3 power supplies, 2 windows' (xp and vista), different power cables and different power sockets in the wall.

Now, all my new hardware is brand new from the box EXCEPT the CPU that is refurbished (after 7 days from original sale, apparently the customer changed his mind).

I don't have enough money to replace the CPU for now, so i would really appreciate any suggestion you guys can give me.

Thanks

[Loopy]

Just so you guys know (and for people using search) i managed to solve the problem. Everything inside was overheating due to the case being completely closed. I've opened one side of the case, and the restarts were gone.
So to anyone with similar problem, you might wanna try this solution.
Cheers!

[SnowDog]

So you're running it with the side off?

[darkXmatt]

i opened my case, but still, it restarts... i have the same prob