system restarts

OK ill try to make a long story short.
I purchased a mobo combo. Asus P5VCD-MX,Intel PentiumD 945 Core2,512mb of Atlas ram, and a Thermaltek fan/heatsink. I assembled these parts into my old case with a HD with programs allready installed.I got the BSOD with a Machine check exception.Deleted the HD thinking something might be conflicting. system the would not restart to load windows(XP home). Sent the combo back to retailer.(combo was asembled and tested by retailer when purchased) Reatailer replaced Ram and sent it back reistalled combo on a blank HD and then got random and frequent restarts. Sent it to geek squad for a diagnostic test. was told they couldnt find the problem,(NO REFUNDS) system would restart before test could finish.Have since purchased new video card Radeon 1650 pro,case with 3 120mm fans,and a new HD. I am getting frequent and random restarts. Restarts during every program that i run. I am Also runing a 550w power supply.Geek Squad did tell me that they would get no video when they installed any other ram they only got video with the ram included. Retailer tech supprt says mobo is fine and that its probably a bootleg copy of windows causing the failure(Its not a bootleg).
Anyone have any other ideas cause this has been going on for 4 months and ive spent too much money on ignorent techs.