mIRC doesnt state that it accepts a directory parameter at all within the /font command, it could very well be that mIRC already knows your 'font directory', C:\windows\fonts\ since it has a list of all the fonts already on your system when you type /font.
So there ideally should be no need for a directory to be specified if your font is placed in your systems font directory, /font -a FONTNAME should work fine.
It might very well be that mIRC is trying to do:
C:\Windows\Fonts\c:\program files\mirc\hypertt.ttf
Eamonn.
