If your pc did pop up as below :
http://star-techcentral.com/tech/story.asp?file=/2006/4/25/prodit/14029720&sec=proditDo this:
It is easy to fix this problem:
1. Use regedit and goto
\HKEY_LOCAL_MACHINES\SOFTWARE
\Microsoft\WindowsNT
\CurrentVersion
\Winlogon\Notify
delete folder "Wgalogon" and all the contents
2. set your system folders to unhide Folder view or you won't see dllcache folder
3. go to c:\windows\system32,
rename WgaTray.exe to WgaTray.exeold
rename Wgalogon.dll to Wgalogon.dllold
4. go to c:\windows\system32\dllcache, rename WgaTray.exe to WgaTray.exeold
5. kill wgatray.exe from TASK MANAGER. Done.
If still have problems, reboot the computer and login to windows xp in safe mode and repeat the above steps