WSUS Database Requirements
Here you will find WSUS Database Requirements for Windows 2000 & Windows 2003 Servers;
On Windows 2000: Install Microsoft SQL Server 2000 Desktop Engine (MSDE) prior to running WSUS Setup & install a security patch, http://go.microsoft.com/fwlink/?LinkId=47363 to MSDE to protect your WSUS server.
For Windows 2003: Although database software is required to install WSUS, it is not listed here because the default WSUS installation on Windows Server 2003 includes Windows SQL Server™ 2000 Desktop Engine (WMSDE) database software.
More on http://wsuswiki/WSUSDB