How to Manually Uninstall SQL Server 2005
Sometimes, you may find that, though you can’t see any SQL Server 2005 instance in the programs list and when you try to install SQL Server 2005, you get a messages saying that there is already a SQL Server instance or some components such as SQL Server Management Studio installed. The reason for this is the existence of corrupted SQL Server instance and you need to remove the SQL Server instance manually.
To do this run the following command from the command prompt:
“%ProgramFiles%Microsoft SQL Server90Setup BootstrapARPWrapper.exe /Remove”
Uninstall the SQL Server components one at a time until all the SQL Server components are uninstalled.
Note: Add or Remove Programs also runs the ARPWrapper.exe program by using the /Remove option. However, the reference to the ARPWrapper.exe program may have been deleted. Make sure that all the services (if they exists) relevant to this SQL Server instance is stopped.