While windows is booting press F8 and select Safe Mode. Boot into Safe Mode and check whether you are getting the login screen with user name icon for logging on. If you can login to the desktop, open Command Prompt with Admin privilages (rightclick on Command prompt command and select Run as administrator).
In the opened command prompt window, type
SFC /SCANNOW
This will start the system file checker and will rectify any corrupted system files. Once this is finished reboot and check.
If the system does not allow you to the desktop in Safe Mode, select Safe Mode with Command Prompt in the boot menu and use the above SFC /SCANNOW command.