If no means of getting a line in to a laptop and it is that important to you, there are external soundcards available, which will at least give a method of getting the sound in there. I think Creative (of Audigy and Soundblaster fame) do one or two Of course, no probs with a desktop.
On the input side, assuming some 'orrible Ghetto blaster with no line out, then the headphone socket will work just as well for most people. Often easier than dismantling the mega Hi Fi if the PC isn't next door to it.
Once you have faffed about and spent valuable hours getting the thing on your hard drive, it really is worth getting some editing software such as the Magix AudioLab, as you can resample, refresh and otherwise clean up the signal, which gives a surprisingly good quality of sound.
If the cassette has been in storage for a long time, remember to fuly wind and rewind a few times.
Conan