In device manager what does it show for display adapter?
something like "generic VGA device"?
because if it shows that
then you need to install Windows 7 drivers
which for the 8400gs you would use the Vista drivers if they
dont make specific 7 drivers
did you try doing an windows update thru Micsrosoft?
they usually have the older cards WDDM driver
that is the driver that MS has in its file server for that device
you are better off with the driver from Nvidia which will usually be newer