First Method
Lauch 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. 
Lauch 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.