I just put together the following system:
Intel Core 2 Duo 6750
Intel Mobo w/ P35 chipset
4GB DDR2-667
WD 500GB HDD
VisionTek Radeon HD 3850
500 Watt Power Supply

Problem: The computer is turning off after 2-3 mins of working.
It started happening right after i changed the RAM from single channel to dual channel by moving one of the sticks into a different DIMM slot (i have 4 slots and I use two 2GB sticks).
It was working fine earlier, when i installed os, drivers, a few games. Any ideas of what it could possibly be?

Try reseating Ram and making sure you are putting it into the right dual channel slots normally the same slot in both grouped so either both to left hand side or both to right hand side. So you have Ram space then Ram space. As long as this is how your mobo supports dual channel. Check you haven't dislodged or broke any other parts in your case?
02coled

Core2DuoE6600ES @3.6ghz
2gb Elipedia 1066mhz5,5,5,15
EVGA 680i
BFG 8800 GTS-OC 320mb
680w Thermaltake Purepower
2 x 250gb Sata
Vista Home Premium

Check particularly you haven't dislodged the CPU fan lead. Turning of after a couple of minutes is indicative of an overheating CPU which will happen very quickly if the CPU fan is not working.

Stuart

List your PSU specs. 500W means nothing....

Otherwise, check your temps in the BIOS & test your RAM with memtest86

event viewer.

I read it wrong and answer it wrong too. So get off my case you peanut.