Use this method only if you DON'T have an NForce MOBO.
1. Go to Add/Remove software and uninstall Nvidia drivers, BUT DO NOT reboot yet, just let that question box sit for a moment while you..
1a. Delete the C:\NVIDIA directory
1b. do a search for nv*.* and delete every file you find. (It will be alot and it won't let you touch 2 or three of them like nv4mini.sys, and nv4inst.dll.
1c. When those files are all gone, then reboot and when the system brings up the found hardware dialogue, cancel out of it without loading any drivers!!!
2. Do another search on nv*.* and delete the last of the files. Just to be sure, empty you recycle bin.
3. Reboot again just to be sure you're starting fresh, again canceling the found new hardware screen without installing drivers.
4. Run the setup program for the new driver series you want to install and go ahead and reboot when prompted.
NVidia is like a cancer when it comes to getting rid of old driver files, and adding new ones without first cleansing your system of the old is almost always bad news. I use the above method EVERY time I switch between driver releases (Which is a serious pain when I want to compare 3-4 versions in a day for perforamnce) But it is the only method I've found that works every time.
Hope this helps.
DWolf