I have done this - moslty with spoken word cassettes - with a cool program called GoldWave. It is free for quite a few uses, then is about $45.00 to buy.

Plugged from the cassette mic output to the mic input on the computer, opened a file in Goldwave, and recorded. Worked fine.

I use it from time to time to record radio interviews from the net, too.

I use freeware program called "Lame" and free GUI called Razor to convert files from .wav to .mp3.

The Goldwave is a pretty full functioned sound editor, too.

http://www.goldwave.com/