Windows SBS 2008 networks define the Windows Update server as the SBS 2008 server. This works well until you decommission the SBS 2008 server, and you need/want your computers to start searching for updates on the internet again. Personally, I've found that as long as you have sufficient bandwidth, I prefer getting updates from the internet over managing/maintaining an internal WSUS server. I'm sure many disagree with me.
The WSUS server is defined in Group Policy. The Group Policy Object on an SBS 2008 network is called "Update Services Common Setting Policy." Here's a screen shot of where you disable the setting (change the value to disabled):