You need to uninstall the previous version first. It may fail if the previous installer was corrupted (like if its files got deleted or something). You need to download the installer for the exact version you currently have and select Remove. I thought you currently have 4.2.1, that's why I suggested you run it and Remove. If you have another one, like 4.1.0 or 4.2.0, find it here:
http://www.oldfoss.com/Classic-Shell.htmlOnce you successfully remove what you currently have, do a reboot, and try installing 4.2.1 cleanly. It should work.