Font Distorted in Address Book, iTunes, Safari- displays as 'A'

I am running Snow Leopard in my iMac and the font displays as 'A' in:

Address Book

Some web pages in Safari

iTunes

Copied and pasted the 'A' text from Addess Book into Word and the font incorrectly displayed is 'Cambria'. However, I had validated this font in Font Book. I really don't know how to fix this.

Any help?