hm, the admins have pretty much hit all the ideas I have. These are unlikely, but worth a try...
turn the power supply off-pull out the motherboard backup battery-put it back in, start compy (you might have already done this to reset the CMOS, I don't know)
check for the video card power requirements, I know that sometimes for processors wattage isn't the only thing that has a minimum.
have you tried Microsoft tech support? you mentioned Vista, and I myself am having severe Vista problems on an HP computer.
are all of your OTHER drivers updated completely/Vista updated completely? I had problems with my hardware before complete driver updates (MoBo included, especially MoBo, or even a BIOS reflash necessary)
also, try HP tech support, since you've contacted ATi and NVIDIA and it seems to be a problem outside of Windows, HP might help...