I guess it is a bit late to answer but I found the thread when searching for a solution to the same problem and thought I should add my solution.
Description:
Had an Nvidia card and then switched to an ATI card for awhile and then went back to the Nvidia card and then couldn't couldn't install drivers for it with the reason being that access is denied after the installer copied the files to the windows folder. Updated the system from SP2 to SP3 as well and the problem persisted.
Solution:
Deleted all nv*.* files in Windows/system32 folder and installed the card again. I tried driver sweeper from guru3d but i could delete the nvidia files even in safe mode, but it was a good help in finding the driver files that should be deleted.