Hello,
I've been having this problem since a very long time now.
My system specs are
Core i7 , Intel DX58BP mobo
3 GB DDR3 RAM [XP]
NVIDIA 9800 GT 1 GB
1 TB HDD and 550w Power supply.

The system freezes while playing games . If it's a graphically extreme game the PC freezes and sometimes reboots in under 10-20 minutes. If I play an older gamer [Warcraft] The system freezes in an hour or so.
I've changed 3 power supplies to date , which temporarily made the system not freeze. I read up on the freezing issue and found out 3 main causes of this
1>Dust
2>H/W problems
3>Overheating / Not enough power

I don't have problem with dust as I clean my PC every three to four days. I checked and rechecked my H/W a lot of times too.
Is it a problem with the Power supply that I need an even greater Voltage one or is it the problem on the motherboard since its Intel , cause a lot of Intel boards have overheating issues. This problem has seriously pissed me off. :(
Please help me!

Are you certain all of your device drivers are up to date. If found those can give you fits exactly as you describe as well.

By up to date... I don't mean what shipped with the product on a CD... I mean based on what every manufaturer has on their websites that will be more current.

Motherboard... Soundcard, video card... etc... ALL of them.

I'll try doing that ! Thanks for the reply :)