Vista, right? It's probably installing into the wrong directory because Vista has this nasty habit of splitting files up and away from the directory they should be in.
Check your current shortcut to the AH skin viewer. Note the directory (the location of the application the shortcut links to). When you install your 2.18 skin viewer update, it should have a first screen with a browse button and a directory listing, right? As long as it's not an automatic thing that skips this step, you can manually tell it to go to the directory you just checked from your AH skin viewer shortcut.