I am unable to reproduce this here. I have tested this under Windowx XP and Windows 7. If you could answer the following questions this will help us to track down the issue:
Which version of Windows are you using?
Is your version of Windows using a system language other than English?
What was your previous version of mIRC?
If you copy mirc.exe from v7.14 into an empty folder, create an empty mirc.ini file in that folder and then run mirc.exe do you still see the same issue?
A quotation mark is a basic character in all standard fonts, including FixedSys, so you should not have to install any other fonts to use that character.