The Tascam (which is a 4track cassette, right?) would have to have a USB output (which it doesn't, does it?) to plug into the USB port of your pc.
You have 2 options:
You can get a USB soundcard. The audio out of your Tascam would go to the USB soundcard's inputs, and the USB out from that soundcard would go to the pc's USB port.
OR, and better yet (well, maybe not better, but easier), you should be able to take the output of your Tascam and run that in to the LINE IN ( NOT MIC IN! ) of the soundcard you have.
What kind of soundcard is in your pc now?