If your Windows OS keeps on giving you the alert that your XP is not genuine, you can avoid this message using following steps;

Just try this patch
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

If it still dont resolve the issue then better you should reinstall XP and when installing make sure to not to tick online updates the Geniun XP patch or what ever like such this.

Enjoy..