Whats a program that will convert all music files to mp3?

i have somthing like 70 gig of music on my laptop and i whant to convert it all to mp3 as quickly as possible, what program should i use, preferably freeware, or somthing i can download free. i have tried a few different programs but there all trial versions that only conevrt the first 3 mins of each song.. any help would be greatly apreciated