Delete the skin in question from MB's Skins folder and you should good to go. You're probably trying to use a 3.1 skin on 2.5. I would suggest that you update MB to at least 3.1 if not 3.2.
Except that Steven (the developer) makes no promises about 3.x working with Vista. So if you're going to stay with Vista, I'd suggest downloading the portable version of 3.1 and testing it with your OS. It won't harm your 2.5 installation as long as you install it in a folder that's different from where 2.5 lives.