Well, for future reference, here's my advice (take or leave, obviously!) if you ever find yourself needing to install or upgrade graphics card drivers again (which is not unlikely, actually) :
* Grab the latest copy of the (in your case) Nvidia drivers, for the correct OS (inc. 32- or 64-bit in particular)
* Also grab the latest version of http://www.guru3d.com/category/driversweeper/, which is free, no strings attached, and safe. Install this now.
* Uninstall whatever Nvidia graphics drivers (inc. PhysX) are officially installed, using Control Panel for instance, without restarting until they are all done, if more than one thing of this kind is present.
* Restart, and go into Safe Mode (using F8 then selecting it, typically)
* Run Driver Sweeper, and check *only* all nVidia graphics and PhysX things, then click "Run" or "OK" or whatever it is. ***
* Restart, and *don't* go into safe mode. Ignore / cancel any OS-prompted stuff about new hardware or whatever.
* Install those latest Nvidia drivers.
* Restart once more, again *not* in safe mode. Should all be working 100% now.
Done!
edit: *** In some people's cases, also check everything ATI (e.g., when switching from ATI to Nvidia) - same goes in reverse, naturally.