Recently I was involved in troubleshooting a SQL Server 2008 installation that was failing.
I found out it was failing because the SQL Server setup was detecting leftover components from previous SQL Server installations.
OK, no problems I thought. Let's just remove these components by using one of these methods:
How to: Uninstall an Existing Instance of SQL Server (Setup)
How to uninstall an instance of SQL Server 2008
SQL Server 2008 installation fails when you try to install SQL Server a second time
These did not work... :(
I remembered there were manual uninstall steps back in SQL Server 2005:
How to manually uninstall an instance of SQL Server 2005
Well, guess what? There are no such "manual uninstall" steps for SQL Server 2008 and SQL Server 2008 R2. There used to exist such manual steps, but they created more problems than what they solved, so they were withdrawn. All that we could really do at that point was to reinstall the Operating System.
What did I learn from this frustrating incident? Save yourself time and frustration and reinstall the Operating System when you are replacing/upgrading your SQL Server. Uninstallation does not always work flawlessly and leftover components issues are not easy to resolve any more. Hope you won't run into the same issues as I did!