hi
drivers dont have anything to do with the pc not booting.
just install the ATI drivers when you get in windows, and they should overlay the old ones.
The real problem is more likely that you havent connected the 12V plug to your graphics card.
most of the ATi cards got these. they are located either on the far end of the card, or on the top. so just take a 12V cable from your power supply and plug it in your graphics card if it need a extra 12v. if not i suspect something is wrong with the card.