You won't be able to see the old drivers on the C drive (one way is to download the old ones, go to run the exe file, and then cancel it after its unpacked
This is because when you run a new graphics file, it just creates a sort of temporary file in C that never sems to go, and the old folder would have been replaced with the new folder, but don't worry.
Just keep the EXE file handy
As for the 77.76 drivers, I found them after searching on google. Did take a while though, so aren't the easiest set to find