WordPress is overwriting my accents into "?" character

Hello all!

I’m having some trouble with my WordPress when I try to use accents. This is happening into the posts section, categories and so on. When I create a post with an accent, for example “Acessórios”, after I click in “Update”, WordPress converts the accent to an interrogation character: “Acess?rios”.

I’m using the eStore theme from elegantthemes.com. I have to resolve a problem with fonts using Cufón in this theme, which don’t accept accents also.

I read a lot of posts in this forum and in the WordPress Brazilian forum. I’ve tried some workarounds but I haven’t found any solution for this. Could you help me, please?