Install SP1a in Win XP Prof.

I have a Domain Windows 2000 with Domain Controllers Windows 2000 Advanced Server. I’m traying to install Service Pack 1a in 100 Windows XP Professional using Group Policy, but nothing. I used this address to do it :

http://www.microsoft.com/WindowsXP/pro/downloads/servicepacks/sp1/spdeploy.asp

But Nothing. I guess the problem is XP.