Well, it MAY still be a power supply issue. Thermaltakes are known to be a little on the shady side when it comes to making PSUs.
That's what it sounds like to me anyways. I doubt your 9700pro card is faulty, because you say it is working before you install the drivers. (Do you have another system you can test it on? A friend's for example?)
When you did the fresh install of windows, you were unable to install the ATI drivers right? If it was a fresh install, this means there were no old nvidia drivers on your system messing you up....
And your 5200 works no problems?
I can think of no other possibilities. See what some others say, but I'd say your PSU may be the cause.