I recently needed to work with SSIS on my laptop with an existing SQL Server 2008 Express instance. I did not setup the service accounts correctly on the SQL Server 2008 Development install--so needed to remove the corrupt install.
There doesn't appear to be much documentation on this—well, if there is, then I didn't key in the best search criteria! Anyway, the removal of a named 2008 instance is an easy set of steps, although nervous-making--since you'll be uninstalling Microsoft SQL Server 2008 without any initial guidance.
Here are the steps you will need to take:
1. Navigate to Control Panel->Add/Remove Programs
2. Select Microsoft SQL Server 2008
3. Select Change/Remove
4. Select Remove
You will be asked for a specific instance. This is where you type in the name of the instance you want to remove. DON'T get click happy on what to de-install. Click ONLY on the items that specify your named instance. LEAVE all the SHARED features. If you delete them, you'll corrupt your other installations on that server.