I ran into an issue where I got a corrupt DBXL installation which would refuse to uninstall. I have removed the virtual directory and the databases manually but Windows Installer information persisted, preventing me from reinstalling DBXL. So I searched and found Microsoft has a nice little tool for exactly this type of situation: the Windows Installer CleanUp Utility. You can find it here:

http://support.microsoft.com/kb/290301

After installing it, simply scroll down to the app that refuses to uninstall and click "Remove". WARNING: This does not uninstall the program but just clears the Windows Installer information. You will have to do all the manual cleanup yourself! Use it at your own risk and only as a last resort solution.