there're many softwares

I have been using MusicMatch Jukebox, in which you can choose FILE->CONVERT. MusicMatch is available for download (the basic version is free) at
http://www.musicmatch.com/download/free/?OS=pc&MODE=input&DID=&OEM=MMD&LANG=&SOURCE=getbasic_*****promo

It may not be the best for other people but worked fine with me.