If you are using Windows, I recommend
GoldWave. Set it for unbounded recording.
Get a male-male (or whatever the official term for that is) cable, one end of which will plug into the Headphones jack of your best tape player (best for best audio extraction quality) and the other end will go into the LINE IN (
not the MIC IN!!!) jack of your sound card. If all goes well, GoldWave should now respond to sound coming in.
After recording, save it. If you are okay with MP3, then just use GoldWave's built-in MP3 converter, but if you want to save it to Ogg Vorbis format (which I strongly recommend due to its superior compression and open-source-ness), you can download a special plugin at that same web site.
P.S. The reason you don't want to plug in the cable into the MIC IN jack of your sound card is that you will get A LOT of noise in the short pauses of your music. This is what I did before I knew better -- had to re-record several tapes.
Good luck.