Yeah, mIRC 7.x changed to %appdata% as that is the path Windows wants apps to follow. It lets multiple Windows logons to have their own individual settings in apps without the app needing to really do anything on its own. Of course, for many home users, that doesn't really offer much benefit as they often only have one user account per computer.
With XP, you probably won't run into any problems with permissions if you use Program Files, but it doesn't hurt to install somewhere else to be safe.
