You can use Audacity to do this. Get a cable that has 3.5mm stereo jack on both ends (stereo-to-stereo cable like this). Connect one end to headphone socket of your music system, and the other to line-in/mic of your PC. Launch Audacity and select "Line-In" or "Mic" as source and click "Record". Play the tape on music system and let Audacity record it. Once done, you can export the recording as MP3 in Audacity.
You can also use RCA-to-stereo cable if your audio system has RCA ports for speaker connection.