I can confirm the INI problems, if you run the same mirc.exe like 3 or 4 times and you get them to perfom the same INI operation on the exact same file, it will crash, easily.
It would appear mirc cannot on windows xp read from files that are currently being read from or written to by other mircs without crashing itself or others in the process, probably due to something outside of your control.
It does not depend on the OS being used, it's the same on windows 7, and it's due to mIRC doing weird things when it cannot open the file or something.
As far as your cpu goes, keep in mind that if I have 4 cores, one mirc.exe runs only on 1 core, so it would use a maximum of 25%.
