If you are running WSUS on Windows Server 2003 and decide to upgrade to Windows Server 2008 you will received compatibility errors unless you are running WSS 3.0 SP1 which includes support for Windows Server 2008 and SQL Server 2008.
It can be downloaded from:
http://www.microsoft.com/Downloads/details.aspx?familyid=F87B4C5E-4161-48AF-9FF8-A96993C688DF&displaylang=en
