Originally posted by Christopher
I used RealPlayer plus to do exactly what you're describing. It is pretty simple: if I remember correctly, I just went to "Tools" and selected "Record from Mic In," ran a line from my cassette player's headphone jack into my "Line In/Mic In" on the computer, and pressed "Play" on my tape player, and "Record" on RealPlayer, and that was it!
I can't remember if it records DIRECTLY into Mp3 format, but if it doesn't, its a simple conversion afterwards.
I used mine to archive a bunch of "White Horse Inn" broadcast tapes that I borrowed from RTS Orlando's library.