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Here's some ideas to try:
1) if you have a basic old PCI video card, then pull your fancy card out and stick the PCI card in. 1MB Cirrus Logic 5446 or whatever. Fire up the system and now it has to adopt the new video card. Go into Control Panel and uninstall the drivers for your nice card, then shut the system down, take the basic card out, put your nice card in, and reinstall its drivers.
2) if you don't have Option #1, then try starting the system in VGA Mode. This is like starting in Safe Mode, you begin hitting the F8 key on the keyboard juuuuust as you expect the first Windows boot-up screen to show up (and if your keyboard has a F-Lock key, hit that first to make the F8 key into an F8 key).
3) I assume you already tried just shutting the system down dead cold and making sure that just the monitor's cable is plugged in, and not the TV cable?