Try booting into Vista's "Safe Mode".

Turn the computer on, and just before Windows begins to load, start tapping the F8 key gently once a second. This should take you to a list of Safe Mode options.

Select the top one, "Safe Mode" and press Enter. Hopefully this will load Windows but only with generic display drivers. Other non-essential drivers are not loaded.

If it loads OK in Safe Mode you can then reboot, (restart), back into Normal Mode and see if that has fixed the problem. If not, return to Safe Mode and see if you can run System Restore to return the system to a previous date/time.