Sometimes, you may find that, though you can't see any SQL Server 2005 instance in the programs list and when try to install the SQL Server 2005, it gives messages saying that there is SQL Server instance or some components like SQL Server Management Studio. Reason for this is existence of corrupted SQL Server instance and you need to remove the SQL Server instance manually.
At a command prompt, run the following command:
"%ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\ARPWrapper.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 exists) relevant to this SQL Server instance is stopped.