Thanks, that helps narrow it down - it's proving to be a tricky issue though as I have not been able to reproduce it yet. Here are the exact steps I took to try to reproduce it:

1. Using Windows 7 32bit, Japanese System Locale.
2. Create an empty folder on the Windows Desktop.
3. Copy mirc.exe for mIRC 6.35 into this folder - and no other files.
4. Create an empty mirc.ini in this folder.
5. Run mIRC.
6. Fill in details in the Connect dialog.
7. In Options/IRC/Messages disable/uncheck the following:
"SJIS/JIS conversion"
"Multibyte display"
"Multibyte editbox"
"UTF-8 display"
"Font linking"
"ANSI codes"
8. Connect to a server on
9. Try the following commands:
//echo 4繝舌・ 7blah in ANSI
//echo 2バー6nbn in UTF-8
//msg $me 4繝舌・ 7blah in ANSI
//msg $me 2バー6nbn in UTF-8

If you follow the above steps exactly, does mIRC still crash for you?