You may be overdriving your input - try dialing the volume out of your cassette deck back, and then boosting it on the computer side if needed; i.e. - balance the input. (Here's another plug for Audacity - it has both input and output VU meters...) Also check to see if you have the treble boosted.
You may also want to try different cables...cables "go bad" for no appearant reason around recording gear.
Also be sure you have the deck's noise reduction engaged on the deck for playback...