This point wasn't really stressed, so I'll chime in here:
If you *do* want to store all your data in a self-contained directory, mIRC can do this with the -portable command-line switch, or if the mirc.ini file is in the same directory (as stated)-- but do not store your self contained mIRC install inside of C:\Program Files or C:\Program Files (x86). You will run into problems, if you try to do this.
Instead, place it somewhere else, as others have suggested.
