How can SQL Server 2005 Instances be manually removed (if uninstall does not clean them up)?
After long searches and trying to delete things from registry I found the following solution:
Delete SQLEXPRESS from HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names
Delete HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.1
Delete entire branch HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\90
To remove actual databases, delete folder: C:\Program Files (x86)\Microsoft SQL Server\MSSQL.1
Disclaimer: Use at your own risk.
Microsoft lists other cleanup steps to also consider on MSDN, besides those few registry keys to fully cleanup an instance of SQL Server 2005 manually - This may also help from a SQL Server 2005 Express standpoint -http://support.microsoft.com/kb/909967
This blog post has an alternate approach that I have not tried