How can I get rid of back ground noise ?

I am using a digital recorder to record professors' lectures for my classes. After loading the .mp3 files on my PC, I found out that in most cases, the back ground noises seriously interfered with the professors' voices ( the humming noise usually heard in recording )

_ Is there a way to reduce these noises ?

_What is the best Audio editor would you recommend for this job ?

_ Is ther such a program that just pick out the human voice ?

Please comment . Thanks .