XP Home randomly reboots

[Chisel N Gel]

I was upgrading my computers mobo and discovered the motherboard was bad and shorted out. Now I have my old ECS P4M800-M7 motherboard back in but with a new processor and powersupply. I now have an Intel pentium D dual core processor in the old ECS along with a new 500w Utlra power suppply. At first my computer was working, then I tried playing a few games. Immediately my computer reboots once the game went full screen and it's been rebooting randomly since.
I've had this problem before and it was with a bad stick of RAM, but that time when my computer started up it told me that the computer had failed a RAM test. This time it's not doing that.

 

[just_a_nobody]

When you say shorted out, are you sure it was not shorted out on an extra, or bent, standoff?

It could still be a RAM problem, a driver problem, a MOBO problem, a software issue, a video problem, almost anything may be the cause. Are you sure the new processor is compatable with your old MOBO?

 

[Chisel N Gel]

No, I'm pretty sure it was just shorted out, but I've ran memtest and nothing was wrong. I also have tried taking out my Radeon 9800 and used my built in motherboard video card, and I still have the same problem. I've also checked my motherboard manual and it says that it's compatable with my new processor. Ever since I took out the new mobo and put in the old stuff windows had trouble with drivers. thanx