I have ran into this problems many a time and getting it to work is a crap shoot game... or so it seems.
Things I have tried to solve my problem... in this order (your miles may very
.- Go to Nvidia and download latest drivers
for my OS (x64 + 181.20) and Nvidia System Tools (6.03 latest version 01-09-2008)
.- Get yourself a good Registry cleaner (I use CCleaner) or use Nvidia Driver
Sweeper (careful, I have never had good outcome with it and I usually end up uninstalling my nForce drivers as well... not funny at all!
1.- Go to Safe Mode
2.- Uninstall latest Nvidia Video Drivers... Do not Reboot
3.- Uninstall PhysX drivers... Do not Reboot
4.- Uninstall latest Nvidia System Tools (whatever Nvidia drivers left you have except for Ethernet or nForce chipset drivers, read option "B
")... Reboot after this to Normal Windows.
5.- Once at your normal desktop, run CCleaner and clean your registry from any Nvidia remants of those drivers you just uninstalled.
6.- Install latest Nvidia drivers (181.20)... remember to "Run as Administrator"... Reboot
7.- Install latest Nvidia System Tools (6.03)... again, "Run as Admin" and Reboot
8.- Go to Device Manager and make sure your Monitor is there with its name and not just as a Generic P&P monitor or Generic Non-P&P monitor... don't ask me why this holds any water, as stated before, I really don't know what causes this error and this is just a temporary band-aid, at least for me.
Good Luck and please, come back to tell us how it went, it may help others as well (aside from making me feel good that I have helped others and not just leech for help myself!