I have released a new version of the MiKTeX uninstaller (aka "Remove MiKTeX! Wizard").
The new uninstaller makes a more thorough job:
- removes the MiKTeX bin directory from the PATH environment variable
- removes all registry settings
- completely removes the installation directory (C:\texmf) (previous versions left some files in the installation directory)
- removes the data directory (C:\localtexmf)
If for some reason you want to keep the data directory, you can always uncheck the "Tidy up thoroughly" check box on the welcome page.