I am pretty sure that the installation check is looking in your registry, and it can't delete the entries there, because it has already deleted the uninstall info.
If you are afraid of messing up your registry (can't blame ya), try using Regcleaner. Let it do a scan of your registry, and it will present you all the redundant registry entries you have. Then all you got to do is click delete.
That should do the trick, and the util shouldn't be that hard to find.
It's only after we've lost everything, that we are free to do anything.