I agree with Mellors. I don't see how pushing your graphics card could corrupt windows, if you have an onboard graphics card try taking your 9800gt out and plugging the vga cable into the onboard card, or try putting your 9800gt into another pci slot, or try another graphics card (if you have one) in the PCI slots or you could put the 9800gt into another pc. Lmao, after you try all these I'm sure you you'll be able to find out what the problem is. :p
As for backing up the data on the hdd, you could just connect it to another pc and transfer the data onto the other pc's hdd, then format and reinstall windows onto your own hdd and put the data back onto it?
I'd go for Windows 7 if you can afford it, although if you still have your vista serial code, I'm sure you could source a disk from a friend or something and use that to do a clean install?