what OS are you using?
Go to:
NVidia GeForce 8400 GS drivers
Choose the version of windows you have installed and download the first one as it will be the latest driver.
Check device mgr to see if any yellow marks present
-Right click on My Computer and choose properties
-go to hardware tab then device manager
Look down list and look for any yellow marks present and then right click on the one that refers to video card (if present) and choose uninstall and say OK
Now close device mgr and go to downloaded file and try to install file downloaded.