Your collection of cassettes might have sentimental value but it has to be admitted that audio quality was never a selling point for tapes. The outcome of this is that there’s not much of point in spending a lot of money on the kit to digitise your mix tapes and convert them to MP3 or CD. Instead, your choice of equipment will be made, primarily, on the grounds of price, availability and ease of use.
If you still have a cassette deck or walkman, the cheapest and easiest solution is to use that. If you’ve not used your player for a while it would be a good idea to use a head cleaning tape (or a cotton bud) first to remove grime from the heads and thereby improve the audio quality. If you don’t have a cassette player, although they’re largely obsolete, you can still buy portable players from around £15 - or look on eBay.
Whatever type of player you choose, you should connect its output to the line-in socket on your PC’s sound card. The exact type of lead will depend on the output socket on your cassette equipment but typically you'll need a 3.5mm minijack to minijack cable. These and other cables are widely available from your local Maplin store or from electronic suppliers online.
Once connected, use Windows' Sound Recorder (search for Sound Recorder in the Start menu). Make sure you select Line In as opposed to Microphone before recording. This will digitise your cassette as a WAV file but if you want an mp3 file, which is much smaller and hence better suited for playing on portable devices, you’ll need to use a conversion utility – look online to find a free one. You could also download Audacity (sourceforge.audacity.net) and the accompanying Lame MP3 encoder, both of which are free.
An alternative, if you don’t already have a means of playing cassettes, is a cassette player with a USB interface which is designed specifically for digitising cassettes. These devices have a built-in analogue to digital converter so they don’t rely on your sound card and are driven using the software provided.
Generally they also allow you to save the file in MP3 format thereby eliminating a separate conversion. This functionality is sometimes provided by integration with iTunes. One supplier that specialises is equipment for digitising analogue audio media is ION Audio. The Tape 2 Go and Tape Express + USB cassette players cost between £25 and £35.
Whatever method of digitising you choose, it would be a good idea to take a good look at your cassettes before spending time digitising them, especially if they’ve not been played for some time.
While dirt probably won’t have worked its way inside the cassette, if there’s dust around the area where the tape is exposed, it could end up inside when you start recording so give it a good clean first. Also, make sure that the spools haven’t seized up, if necessary turning them manually (a hexagonal pencil is ideal for this) to get them moving before trying to play them.