A real quick way to create a windows update server. (WSUS 3.0)

These steps are for windows server 2003

• Turn on Microsoft Internet Information Services (IIS) 6.0 (IIS 7.0 on server 2008)
• Microsoft .NET Framework Version 2.0 Redistributable Package or higher (http://go.microsoft.com/fwlink/?LinkId=68935)
• Microsoft Management Console 3.0 for Windows Server 2003 (KB907265)(http://go.microsoft.com/fwlink/?LinkId=70412)
• Microsoft Report Viewer Redistributable 2005 or higher (http://go.microsoft.com/fwlink/?LinkId=70410)
• Now grab a coping of WSUS 3.0 and install(http://go.microsoft.com/fwlink/?LinkId=74472)

Your going to be prompted with the places to install wsus, updates, and the database. If your over 500 clients you are suposed to use MS SQL 2005 server.
Now on the client computers you want to run a reg script that looks something like this

The system will be categorized as the created group called staff. Server side targeting must be turned on at the server. Don't forget to change the ip address to match your server.

full info here: http://technet.microsoft.com/en-us/library/cc708521(WS.10).aspx