Either install them on individual User logins, or else insert a blank mIRC.ini file (or not blank, if you prefer) *INTO* the mIRC installation folder(s). This will force mIRC to save things to the installation folder instead of the user folder. You can also use command line switches to force the use of different settings files, though I personally think that's too much effort.
Note that you will probably need full security privledges for every mIRC folder (and subfolders) if you choose to put the mIRC.ini into those folders. Otherwise, Vista may complain whenever you try saving any settings or downloads or whatever else. Whether or not you have to do that depends on where those folders are located. For example, you already have full permission if you were to install it to a folder on the desktop.