Make certain you are happy that everything in the MC26 library folder, specifically, things like the cover art folder (if that's where you have it) have properly copied across to MC28. Don't check using MC28. Check the setting in MC 28, then use explorer to check that everything was properly copied across, and then, when you're satisfied...
Uninstall normally using Windows 'add/remove programs', choose to uninstall everything, and then reinstall MC28.
Sometimes, uninstalling may break something in the new version. A simple reinstall will correct any of that.