It seems to me that I noticed an issue in Vista/7 with installing Portable to the Program Files folder. Due to the restrictions on writing to these folders, it caused mIRC to write to the AppData folder even in Portable mode. Installing to another location, such as c:\mirc\ should work fine, or just use a USB drive. I saw this a year or two ago, so it's possible I'm remembering it incorrectly, but I think that's what happened.
