It is important to always back up your system before installing updates. Can you get to safe mode with the command prompt? If so, do the following...
As you probably know, F8 will take you to the screen where you select which mode to login to, select Safe Mode with command prompt. Login as the Admin. and type the following.
%systemroot%\system32\restore\rstrui.exe
Follow the instructions and so forth for restoring your computer back to a good configuration.