Are you getting login screen and if you try to login it is not logging in?If yes it may be due to the missing or corrupted userinit.exe which is present in windows\system 32 folder.Try by copying the same from a working PC to this PC.(For this you may connect this pc hard drive in a working PC,)
If you are not getting login screen it may be due to the corruption of winlogon.exe.Try copying from some other working pc...