Re-reading your question I suspect that you are feeding line output from your PC to mic input on your cassette recorder. On my JVC tape deck I got good results by recording with Dolby and playback without. If you don't have Dolby your best bet is to simulate it manually by increasing the treble during record, etc. It boosts the treble when recording and turns it down on playback reducing the hiss and improving the S/N ratio. First, make sure you've got Dolby noise reduction on, if it's available.
