It's from the character set on your forum; likely the posts where made with the old character set "iso-8859-1" but the default as of 7.5.9 is "UTF-8", basically the data in the database is one character set whereas it's being read out as another, and the two allow different types of characters. You can either suffer (muahaha) with the old posts having some quirks, or change the character set via the language files to what they where before.
If you feel like a little reading, I'd check out
The Absolute Minimum Every Software...Unicode and Character Sets (No Excuses!).
Edit: I've added an entry to the Wiki:
How to Fix Black Diamonds
