Randon crashes-will not reboot until power cord is unplugged

[mad maximus]

Computer is experiencing random, sudden crashes. Push the power button to reboot - nothing. If I manually unplug the power cord, replug in, the computer will reboot. Get the screen about the improper shut down, select last good start up and the computer loads as it is supposed to. Tried contacting the manufacturer, but have had no response (so much for superior support from a boutique builder).

Anyone have any insight as to the cause of the crashes and how to prevent in the future?

Computer Info as follows:

Dual boot (XP Pro & Win 7 Pro) MaxForce PC
Thermaltake 500W power supply
Intel Core i7 870, 2.93 GHz, 8 MB
Gigabyte GA-H57M-USB3 motherboard
Super Talent DDR3-1333, 8 GB (2 x 4G) memory
Hard Drive #1: Western Digital Caviar Black WD6401AALS, 640 GB, SATA 2, 32 MB (XP Pro)
Hard Drive #2: Western Digital VelociRaptor WD3000HLFS, 300 GB (Win 7 Pro)

 

[Layback Bear]

Do you monitor the temps of your cpu and gpu. If so what are the temp?
Idle and under load.

What video card do you have?

Please do this also it will help our BSOD experts.

http://www.sevenforums.com/bsod-help-support/96879-blue-screen-death-bsod-posting-instructions.html

 

[mad maximus]

Graphics card is a nVidia GeForce GT 430. At idle GPU is about 83F, with multiple windows and programs open, and playing Skyrim on Steam temp went up to about 155F.