Disable and Remove Windows Genuine Advantage Notifications Nag Screen

First Method
1.	Lauch Windows Task Manager. 
2.	End wgatray.exe process in Task Manager. 
3.	Restart Windows XP in Safe Mode. 
4.	Delete WgaTray.exe from c:\Windows\System32. 
5.	Delete WgaTray.exe from c:\Windows\System32\dllcache. 
6.	Lauch RegEdit. 
7.	Browse to the following location:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\Winlogon\Notify 
8.	Delete the folder ‘WgaLogon’ and all its contents 
9.	Reboot Windows XP. 


Second Method
Another alternative suggested by dman is by using System Restore to restore the PC to a previous restore point that WGA Notifications hasn’t kicked in, and then carefully stop KB905474 from been applied to the system. To use System Restore, go to Start -> All Programs -> Accessories -> System Tools -> System Restore.
Note: Again, you may be prompted again to install WGA Notification, so it must be bypassed.