Sometimes windows gets its panties in a wad and refuses to believe that the settings are right. Download the correct drivers for your card and remember the directory they are in. Make sure that they are ready to go. Unzipped ect.
Boot into safe mode and go to device manager and make sure that there is only one video adapter listed. If there is more than one remove them both and reboot. If you have to do this you might get lucky and on reboot it will detect and you can install the drivers and get on with it. If win tells you that the drivers already there, tell it to use the new ones. This will replace corrupted files. If this doesn't solve your problem read on.
Set your video to standard vga and SHUTDOWN!Don't reboot it doesn't always flush memory when you reboot. A shutdown guarantees to flush the corrupted drivers out. On reboot you might get a I know better than you do window about the display settings being wrong. Cancel out no matter how much it insists. And be patient. It sometimes will take a while as windows ponders the situation. If it has the display manager open great if not go into control panels and open display and go through the procedure to install new drivers for the adapter. Direct it to the location you saved them in and even if it thinks it know's better insist on using the new drivers. If it says that the files are already on the hard drive tell it to replace them. On reboot you should be able to go in and set display colors and resolution.
Good Luck Lee
I have had to go as far as going into resedit and ripping the drivers out by hand before doing a reinstall.