Cant install drivers for my nvidia geforce 8400gs!

and BTW, if you want to revert back to motherboard gfx (hope you don’t have to :) ), then just use your motherboard installation CD to restore the graphics driver for the integrated chipset in your motherboard.
I had the same issue when I first bought a gfx card (9600gt). I followed the procedures I mentioned in my previous post and everything worked normal after that. Hope it works out for you too :)