Have you turned off automatic install as explained here: Drivers - Turn Off Automatic Driver Installation
When you uninstall the old drivers are you doing it from Control Panel > Remove Programs?
Uninstall PhysX and 3D first, then the driver. The control panel will go with the driver.
You will be asked to restart. Don't.
Next, go into Windows Explorer and delete the entire C:\NVIDIA folder.
On restart, Windows 7 will find your video card and install a generic WDDM1.1 driver. Let it. You will be asked to restart. Do it.
Once back on the desktop you can now install the nVidia driver version you want.
Hope that helps.