I've done this using a programme called Musicmatch, but I'm sure there must be others about. I did it using a lead from the headphone jack to the line in socket, and a bit - well a lot actually - of trial an error in gettings the output and input levels right. However, once you find a good balance, that's it.
But, unlike CD's which can be ripped in seconds, with tapes you HAVE to do it in real time. I generally recorded a whole side at once, and once done, further edited it down into MP3's - for that I used a programme called Mixmeister, but I think windows has something, and I think Nero does it too..
Though the software I used wasn't exactly dedicated to the purpose, I honestly think I did it the best way possible. It was a LONG job though (I had loads of tapes) and I wouldn't like to do it again.
It might be an idea, if you didn't need all the tracks on a tape converted, to just roughly do the one's you do want, and then edit them.