Computer Problems - Game related

Hello, now for the past 2 years or so a reoccurring problem has been happening with my computer, I've found it impossible to play games like The Sims 2, Guild wars, The Elder Scrolls III: Morrowind and The Movies. First the game runs for at least 20 seconds, and then the computer reboots. Just 2 months ago, I've began to have problems with the Hardware and OS (Windows XP home edition), and I've had to reformat my system at least 3 times, within the past 2 months. I've also had problems with the screen blanking out, which occured once again just last week, to fix it, I cleaned out the Heatsink and reattached it. Now, just yesterday I reapplied some thermal grease to help it run better, but it still fails.
Now I've installed ASUS PC probe II and the only overheating warnings I get are the Motherboard Temperature(45 Celsius) and the CPU(Unsure.). Now, after running Morrowind I've found out that the problem is in the Motherboard temperature. This problem is slowly getting worse and I'm concerned that it might soon result in a total system failure.

The Hardware in my PC is:
ASUS P5B Standard Motherboard
Pentium 4 3.2 GHZ
2GB RAM
ASUS 6600 GT graphics card

Please help.

 

I've installed two more fans and sealed some holes and the temperature has dropped, now PC Probe isn't going off when I play Morrowind but it's still shutting down.

 

try opening the case and blowing a fan at the video card(standard household fan) to see if that helps. It sounds like your video may be overheating. Also Make sure of cpu temps.If that does help then I would check into a new card. What size power supply do you have? and how many amps are on the 12v rail?(should be listed on the psu somewhere)

 

Well, I'm going to blow a household fan in, I hope it helps. And I can't find the amp count on my Power supply. And size?

 

OK, I tried a household fan and it didn't help.