Downloading MSDE
You can download the MSDE database engine from Microsoft. We recommend you search their site for the most recent version. Here are several links to try:
SQL Server 2005 Express Edition
http://www.microsoft.com/Sqlserver/2005/en/us/express.aspx
SQL Server 2005 Express Edition Download
http://www.microsoft.com/Sqlserver/2005/en/us/express-down.aspx
SQL Server 2008 Express
http://www.microsoft.com/express/sql/download/