Actually, it has nothing to do with mIRC at all. It is a Windows (I use XP Pro SP1 but I'm fairly sure that Win2K has something similar) setting:
> Start
> Settings
> Control Panel
> Regional and Language Options
> Languages Tab
Supplemental language supportMost languages are installed by default. To install additional languages,
select the appropriate check box below.
[ ] Install files for complex script and right-to-left languages (including Thai)
If that checkbox is checked, the Ctrl + [Left|Right] Shift works.
NOTE: If you change that setting, Windows will uninstall all support for Arabic, Armenian, Georgian, Hebrew, Indic, Thai and Vietnamese and will require a
reboot. However, you will not be vexed with the right-to-left text input problem any more.