Since many days I had this issue with my Mac G5 Desktop, when I view few sites or yahoo messenger I wasn't able to read anything as it appeared like alien characters. Screenshot Here but in my macbook pro this problem wasn't there and not even in my pc.
Now I found a solution through another great site thought I will share this with you guys, credit goes to individual contributor of that particular site, especially user acescence
QBN Solution This is a great site for designers can learn many things from this site.
I found solution at Apple also Apple Solution
May be someone else in DT is having this issue, I hope you get an solution.
Bottom Line: Helvetica Fractions font may cause text to display improperly in Address Book, iChat, Safari, Mail (heading from Apple site)