International characters wrong into email text for orders

[Batusai]

Hello.

I'd like to know where to find the file with the text for the e-mail sent to the customer after they make an order.

The email I receive is OK but the customer receives a text with strange characters where "International characters" should be shown. For example the Spanish "ñ" or stressed vowels á, é, í, ó or ú.
I happens in the subject and in the body itself.

I've read it might be the /admin/tabs/AdminOrders.php file but it's not very clear to me. Is it the right file?
I've edited that file a few times now, but I can't remember if I ever saved it with UTF-8 without BOM encoding.
I've seen the text into /mails/es/order_conf.html but I cannot see the subject so I guess the problem is elsewhere.

Thank you in advance.