Yes, mIRC has always required administrative access since it saves settings in the mIRC folder where mIRC is installed. This has made it portable and allowed users to install multiple independent copies of mIRC as well as share/move/test installations easily.
Under Vista however, any application that tries to store data in its own application folder, eg. "Program Files\mIRC" will be virtualized and will store data in a Virtual data folder that is managed by Vista.
Because of this change in Vista, the next version of mIRC will have to store all data in "Application Data\mIRC", separately from the mIRC executable. Unfortunately, the main effect of this will be that mIRC users will no longer be able to run multiple independent copies of mIRC, or share/move/test installations, as easily as they have in the past.
I'm still thinking about the best way to implement this change in the next version and how to migrate all user data in "Program Files\mIRC" to "Application Data\mIRC" for existing users.