So then I am able to uninstall one copy, but now I have a hosed up copy that I can't get rid of.<P>When I try to uninstall it, it tells me that it cannot find "uninst.isu" so uninstallation will ...

Even worse, if the system databases are using new features (for example, partitioned tables in master databases), this could prevent SQL Server instances from starting after you uninstall SQL Server ...