Which audio file converter program do you find to be the best?

I'd like to be able to convert my mp3's into wma files. I'm looking for something which is user-friendly, relatively inexpensive (free is always awesome of course) and which can do both m conversions as well as one by one conversions. I'm computer literate and learn pretty quick but I'm definitely far from being as knowledgeable as someone who went to school and study computer information systems or a related IT field.