OpenCodecs installer fails if registry contains entry for a previous installation but files no longer exist
Sometimes, it can happen that a system will end up with a registry entry for the OpenCodecs uninstaller, but the uninstall files are missing or gone. If this happens, the installer will refuse to install. It prompts to uninstall, then fails to run the uninstaller. This can be worked around by using CCleaner or something to delete the uninstall entry, but it's still a problem.
The installer should clear out the registry key for the uninstaller if the uninstall file is missing and proceed to installation.