Google has just added Persian/Farsi language in its free translation tool which means you can now easily translate English-Persian/Persian-English instantly. Here is the link to try it out:
Google Persian Translate
Here is also a link in Persian with step by step instructions on how to use the translator:
Aftabgardan - مسرتبخشترین خبر IT : گوگل و FaceBook امکان ترجمه ...
Here is also the screenshot of English-Persian Translator