Shells Missing?

I recently updated XP. After the update installed, my theme, GSM, no longer was showing as one of the theme options. I then downloaded Replacer and tried to run it, but didn't understand which files to use and closed it without doing anything. I then reinstallled the UX package and the theme. The theme now shows up as one of the options, but the preview is black. When I try to apply that theme, or even one of the standard windows themes, I keep getting a message saying that the shell file is missing. What do I need to do in order to enable theme changing again? Thank you!