Just something I came across, thought would be helpful in future...
Kill the process wgatray.exe in Windows TaskManager (Select it then right click and select End task) and restart Windows XP in safe mode. Now delete the following files:
Delete WgaTray.exe from c:\windowss\system32
Delete WgaTray.exe from c:\windows\system32\dllcache
Start Windows Registry editor and delete the folder "WGALOGON" located in the following location:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ Windows NT\CurrentVersion\WinlogonNotify.
Delete all references in your registry to WgaTray.exe (repeat search using F3 function key).