Try shutting down your computer, and when the OEM's logo comes up wait until the screen goes blank, then hold down the F8 key and select Safe Mode, when the login screen appears, try logging in with your normal account, if you have no luck, use the built in Administrator account and try to recover any lost data. After you have backed everything up, try a system restore and if you have no luck then Boot from the Windows 7 DVD and select repair my computer during install, this utility won't erase anything, select repair and if it doesn't work after all of this, re-install windows 7.
Hope this helps, Jack