Perhaps this post will help:
Installing Party Skins.
But if you don't know what you're doing with the Registry, you can easily hose your machine by mucking with it. Don't do it without being certain you understand
exactly what the steps mean.
Easiest is to open your account on another computer that has never had Party or a skin installed, then install the software on your computer, and log in. Once you have an account, you should be fine; it only checks for other skins when you try to register a new account (at least, that's my experience; I can run Party and Empire with no problems).
-Mike