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I think you can fix it in Safe Mode. These instructions are for Windows XP, but Windows Vista is very similar.
1. Wait until the disk light hasn't gone on for at least a minute, then press and hold the power-off button. (This is to increase the chances that the computer isn't writing anything to the disk when the power goes off.)
2. Reboot the machine and press the F8 key repeatedly.
3.Select Safe Mode from the resulting menu.
4.The machine will continue booting, but the Windows desktop will look different.
5. Right-click on the My Computer Icon and select Properties.
6. Click the Hardware tab.
7. Click the Device manager button.
8. Find the video card and enable it again.
9. OK out of everything.
10.When you're finished doing what you need to do, log out and reboot back into normal mode.
Hope that helps.