I think Steve is on to something.
Download and install from here:
Windows Installer Cleanup Utility
Description of the Windows Installer CleanUp Utility
After it's installed, run it (shortcut should be on your Start menu). The utility will display a list of programs installed through Windows Installer. If you find any entries for Blackberry Desktop Software in the "Installed Products" listing, select it/them, then hit "Remove" and let the cleanup utility do its thing.
Then you can try again to uninstall the Desktop Manager. Once you get it uninstalled, you can install again with the correct options.