Thanks, I was able to reproduce it. After closing the options dialog, I copied the //echo command and pasted it into the Status Window editbox and pressed the enter key. I then opened the font dialog, changed the font from FixedSys Japanese script (which was the default) to Georgia Western script, and pressed OK. At that point, if I highlighted the text (which was now Mojibake) in a certain way, mIRC would freeze.

I tested the Unicode beta version of mIRC and could not reproduce it, so it appears to be a non-unicode multi-byte issue. Thanks for helping to track it down.