There is some haziness about what the best way is to uninstall the Reporting Services from a SQL 2005 installation. Here is the very short how-to on a Windows Server 2008 server!
1. Go to Control Panel and open Programs and Features
2. Select Microsoft SQL Server 2005 or the Microsoft SQL Server 2005 (64-bit) and click on Change
3. Select the “MSSQLSERVER: Database Engine, Reporting Services”
4. Again select “Reporting Services”
5. Click on Next, Next and then select “Remove Microsoft SQL Server“.
This is the very confusing part of the uninstaller, because clicking on “Remove Microsoft SQL Server” will not remove the complete SQL installation but only the option you selected in step 4.
After this the uninstall should have been successful and you will be able to install it again!