You need a free downloadable program called 'Audacity'. Easy to find with google. Its at http://audacity.sourceforge.net/
Connect your cassette output to your computer's audio input and audacity will digitise it into a file. Audacity can then be used to 'clean up' this audio file, cutting down any hiss, and even going some way to removing background noises like the sound of a 'fridge' etc. You can bring out more treble or bass with a graphic equaliser, snip out intermittent sounds like doors opening etc, and rearrange the order of songs. It will output the result in a variety of file formats like 'wav' or 'mp3' and many others. Everything you do is reversible, so you can experiment until you get the result you want.
Not too hard to use, completely free, and very powerful.