How to remove sql instance [closed]

Verify that there is really only one entry in Add/Remove Programs for SQL Server. I have multiple, one for every version of SQL Server I have installed:

If there really is only one entry in Control Panel, then it's possible you have some kind of corrupted install. Microsoft has a guide here for 2005, which may still be helpful in more modern versions (it's unclear from your question which version the old instance is). I've written about removing an expired instance of SQL Server before; this would be similar. Find the GUIDs listed under Uninstall in the following registry key (this may take some manual work) that do not involve SQL Server 2012:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\

For example, here is the entry for the 2008 database engine services on my machine:

For each one, copy the display name (in a comment) and the GUID file (behind msiexec /x) to a .bat file, e.g.

REM SQL Server 2008 Database Engine Servicesmsiexec /x "{32445783-54FC-4AA7-A51A-3B528E8A5E3B}"

The number of entries you'll have will depend on the version, edition, selected features, etc. Once you've copied them all, save the .bat file and double-click it. Depending on the order of the entries in the file, you may get some "unexpected error" results, or potentially errors like this one:

If you get any of the former, you should run the batch file again.

Of course, if you don't want to risk any damage to your system, you could always just disable the SQLEXPRESS instance's services using SQL Server Configuration Manager, and pretend it's not installed.