1.Back up your registry (make a system restore point in case something happens).
2.Reboot windows in safe mode(hold f8 key)
3.Run: registry edit  regedit.exe  it opens and search for  wgalogon  folder and delete.(Just go to edit tab and find).
4.Go to My Computer and click on search tab and find  wga  files and delete.
5.Then again Run :  c:\Windows\system32? look for  wgatray.exe  and  wgalogon.dll  and delete.
6. Run:  dllcache  and look for  wgatray.exe  and  wgalogon.dll  and delete. Just turn off and restart. ALL DONE.