From here

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 to select Report Server and the option to remove it.
6. Reinstall Reporting Services by running the installer on the SQL Server 2005 disc.
7. Once setup is complete, launch the Reporting Services Configuration tool with the Configuration Tools menu in your Start menu.
8. Fix any red-X items in the list.
9. You should be good to go!