Can you describe which version you now have installed and in which folder you've installed that version?
Please also check the contents of those registry keys:
HKEY_CURRENT_USER\SOFTWARE\Florian Heidenreich\Mp3tag\Install\InstDir
HKEY_CURRENT_USER\SOFTWARE\WOW6432Node\Florian Heidenreich\Mp3tag\Install\InstDir
HKEY_LOCAL_MACHINE\SOFTWARE\Florian Heidenreich\Mp3tag\Install\InstDir
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Florian Heidenreich\Mp3tag\Install\InstDir
It's no problem, if one of those doesn't exist.