I have just read about Windows Server Update Services which is seemingly a free service for Server 2003 and has a client for each XP machine. Not sure which is the best route to take though.
SUS is a very good route in terms of bandwidth, allowing you to have local mirrors of windows update.
The commercial version also allows you to use the same system to distribute other software of your own choice to the clients.
Regards Henrik
Thanks Henrik. :-)