Your cassette recorder should hopefully be a hi-fi quality stereo unit, i.e. has a pair of Phono sockets (RCA as they are known in the USA) for recording and another pair for playback.
To record from cassette into the computer, you will need a wave recorder. Windows sound recorder will do a reasonable job. Audiograbber from www.audiograbber.com will do a better one!
Simply play the cassette into the 'Line In' jack on the sound card, then record the analogue signal as a .wav file. This canthen be played back in Windows Media Player, Winamp, RealPlayer or whatever. To record back to cassette, remove the speakers from the sound card, and connect your twin phono lead the other way round. i.e. from Line Out on the sound card to 'record' on the cassette deck. Press record on the cassette deck and play your wav back out in your preferred player.
it's as simple as that!
Incidentally, I don't use speakers on my PC. Instead, I connect the line out on my sound card to one of the Aux inputs on my rather sexy Denon audio system. It's simply a matter of selecting Aux on the amplifier, and recording using the cassette deck!