After installing Visual Studio 2010 I, inadvertently ended up with SQL Server 2008 Express having already installed SQL Server 2008 Developer. Only when I was going to upgrade to SQL Server 2008R2 I found out I had two instances installed.
I looked everywhere for a place to remove the SQLEXPRESS instance but couldn’t find it.
Only when I decided to uninstall the entire SQL Server suite and start over I found how to do it.
On Windows 7, go to Programs and Features and choose Microsoft SQL Server 2008 R2 (64-bit) (or whatever your version of SQL Server you want to add or remove and instance from).
Choose Remove (or Add, if you want to add a new instance).
Follow the steps and choose which instance you want to remove.
Select the features of that instance you want to remove (select all to remove the instance).
It’s probably something I should have known, but I didn’t and it wasn’t easy to find.