This question has been answered several times this week on the forums:
You should download and run the FontFixer on the same page as the upgrade.
You may need to run it twice and to restart the Mac, in the case of Snow Leopard.
If the FontFixer does not correct the problem, I recommend opening FontBook and looking at Helena per the instructions in the Accordance Fonts PDF. It is probably accessing either old copies of Helena or bitmaps.