Launch windows Task Manager
End wgatray.exe process in Task manager
Restart windows xp in safe mode
Delete WgaTray.exe from c:\Windows\System32
Delete WgaTray.exe from c:\Windows\System32\dllcache
Launch RegEdit
Browse to the following location:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify
Delete the folder 'WgaLogon' and all its contents
Reboot Windows xp