You need a piece of sound editing software.
I use a program called Cool Edit 2000. It was written by a company called Syntrillium, and this "smaller" version of their "studio" product came cheap at $30. That was of course, several years ago. They were since bought out by Adobe and the product is called Audition. The bad news is that they seem to have dropped the smaller version, as audition retails for $170.
I think programs like EZCD Creator and Nero offer some kind of sound aquisition/editing tools. Bottom line, you need to aquire sound from your external device (tape deck) onto your PC and be able to save it into a known file format, preferably .WAV
Search around out there and I'm sure you'll find something comparable to Cool Edit 2000. Once you get there, we can resume the lesson. If you have a real tough time finding an alternative, let me know that as well.