On your cassette player use the line out or the headphone jack. Plug in a 1/8" or 1/4" cable ( which ever one it has and make sure its a stereo cable) into it and plug the other end to the PC line-in which is on the audio card on the back of the PC. cable usually its color coded blue. it also had a symbol that has an arrow pointing to the port it looks like this. even if you dont have an audio card the onboard card will look the same. If you dont have a line in then the only other option is the mic-in but you dont want to use that it will sound ugly and it will be in mono or you can buy a card that has line-in. Then you will ned some software to import it. http://www.nowsmart.com/arwizard/ this program works good for recording any sound that come through audio card. $24 is not bad and its simple. just read what it can do on the website.
If you don't have a sound card, then you can find an audio/usb in fairly cheap. Belkin makes one. I have a Mac and use iMic which came with some decent software as well.
I used a similar one just a while back to record a tape my niece made to CD. I used a Tascam MF-P01 4-tracker and plugged it into the capture device and straight into my USB port. I imported the tracks into Vegas 5.0 and cleaned them up. You don't have to use a multitracker, though. Any good tape deck with Dolby will do the trick.