Video card swap problems

Hello I am having a problem with these cards. I bought a 7900gt an upgrade from my radeon 9200(agp) i have a asrock dual vsta board if your wondering. So i followed the video card faq at [H] and did that. Now i uninstalled the radeon and when i try to go on xp i get a bsod error. So i went in safe mode and installed the 7900gt and restarted back to xp same error. Uninstalled it did it again didnt work... So i have vista oin the other hard drive and didnt have the 9200 installed on it and installed the 7900gt on it thats what im using now. ok so i just formateed and reinstalled xp. It works great with the card on the question mark thing on device manager, but ounce its installed i get problems. When i change the resolution or color wuality it goes to a black screen and my monitor goes on standby after. When i install the nvidia drivers it gets worst, the compter is really slow it refreshes itself slower than now and it just randomly turns off.
PS: now the vista is not on my boot list so its hard to work so can someone

can it be a overheating or power supply problem. o and my screen is weird. when i scroll in internet it goes really slow like a picture at a time.

MY specs are:
asrock dual 775-vsta
C2D e6300
1gb ddr ram
7900gt
540 watt enermax ps
1 250gb hd 1 160 gb hard drive.


Any help is appreciated