When you perform a Basic installation, SQL Server 2005 Express Edition is automatically installed. When you perform an Advanced installation on a stand-alone computer that already has SQL Server installed, ensure that the computer meets the hardware and software requirements for a database server.
Because of Windows licensing restrictions, if you are using Windows Server 2003 Web Edition in a single-server environment, you can only perform a Basic installation. This is because the full SQL Server editions cannot be installed on Windows Server 2003 Web Edition. However, you can install SQL Server 2005 Express Edition or SQL Server 2000 Desktop Engine (Windows) (WMSDE).
SQL Server 2005 Express Edition应该是免费的吧。。。