I don't know if this applies to individual computers or primarily companies, but apparently MS provided a tool to disable the automatic updates for companies that wanted to test it with their applications before using it on all their machines. However, evidently there was a time limit on that - after April 12, SP2 will be automatically downloaded to all XP computers via Automatic Updates.
There's a link for more info - I didn't check it out - here it is:
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/sp2aumng.mspx
