It's very likely that this is due to the RichEdit editbox, since mIRC has very little control over the way the RichEdit editbox displays text. mIRC uses an older version of the RichEdit editbox (riched20.dll) which is consistent across all versions of Windows. However this means that it may have less functionality than the newer versions of RichEdit. I will make a change in the next version of mIRC so that it loads the newer version (msftedit.dll) under XP and Vista - it should have better support for displaying a wider range of fonts.