From here
[http://mtgcsharpguy.blogspot.com/2008/06/properly-reinstalling-sql-server-2005.html]
1. Stop the default IIS web site.
2. DO NOT SHUTDOWN IIS.
3. Delete both SQL Server 2005 Report Server databases.
4. In Control Panel, click on the Change button for the pertinent SQL Server
2005 install instance.
5. Continue