Can't install SSRS 2008 R2 default instance
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I have a SQL Server 2008 R2 that had Reporting Services included and uninstalled. Default Instance.  I now need to add Reporting Services to the installation and I am getting an "Instance Name in use" when I try to install the default instance.  A named instance will not work in this case.

I checked the following registry location for any keys and it just has the "SQL" folder.  No "RS" folder.

HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\

So what I am really trying to figure out is where the installer is looking to see if the default instance already exists becasue that was obviously not cleaned up correctly on the uninstall.  