The computer is hanging at the login screen? Restore your system to a time you know it worked.
http://windows.microsoft.com/en-us/windows7/products/features/system-restore
You will have to use a repair disk. Set your boot order to boot off of the CD drive.
That feature is in your BIOS. make sure to save the settings when you exit.
wit the CD in the drive the computer should ask you to Hit Any key to boot Off Of CD.
Hit Enter. From there follow the steps on the disk.
You can use another computer to make a repair disk.
You could also get a friends.
If that does not work try booting up in Safe mode.
When the computer starts, start hitting F8.
When the menu comes up select Safe Mode.
Once you get to the desktop run a restore.
When the...