I will tell you what to do, no need for any tools or playing with the registry.

First of all restart or start your PC and press 'F8' to get a menu, choose from there 'Safe Mode'

After that navigate to the following folder when you windows starts:
C:\windows\system32

Locate these files:
WgaLogon.dll
WgaLogon.exe
and any other files [normally one additional file that starts with WGA] and rename these files to something like WgaLogon.bak

Restart your PC and that's it ... Viola, you problem should be solved.

To avoid this happening in the future, from the Run type: services.msc and locate the following service 'Automatic LiveUpdate Scheduler', Disable and stop the service.

Never update your windows after that ...
Cheers and let me know if you need any further assistants ...