If you're still able to use the 1.6.x version then it must not have uninstalled. But yeah, that's annoying. The association somehow gets stuck with the version you installed (the "Windows Installer" download) but then if you want to have multiple other IDE versions installed you need to use the "Windows ZIP file for non admin install" instead.
By 1.8.6 do you mean you're using the hourly build (the latest release is 1.8.5)?