You may use any direct sound recording software including i-Sound WMA/MP3 Sound Recorder 6.55 Professional to record sound from your sound card.

I would use free sound editor Audacity. Try it and see whether there is any difference for you.

You also need an audio cable that will connect your cassette player with the sound card of your PC. If you like to record music this way there might be a problem with quality. Use line out of your player, it couldn't produce to string a signal. Or you can play with the volume (use low volume and you won't have any problem).

In practise it is very hard to damage your sound card. However I really managed to do that once