I've been using Magix AudioCleaningLab for 4years with great results:
http://site.magix.net/english-us/home/music/audio-cleaning-lab-10-e-version/ I bought the original CD-Rom version ($50), but it is now available as a downloadable version for $30.
While you won't be able to make analog cassettes and LPs sound
exactly like "24-bit digitally remastered" sources, you can get close enough to fool the ear in most listening situations, (and the cassettes can definitely be made to sound better than they do now). You won't be able to do much about drop-outs other than to edit them out. All effects are applied in real time to the wav file.
You can add a fade in and out to each cassette track to minimize the instant hiss at the beginning and end of the cassette tracks. If your cassettes were recorded with Dolby noise reduction, then you should play them back with Dolby turned on for the most
accurate reproduction (not the way ThomasB described), you can always adjust the sound with the built-in 12 band equalizer. Be conservative with the filters as they can introduce undesirable effects if pushed too far.
One last tip, Aclab wants you to use their Frauenhofer mp3 format and they only give you a limited amount of free conversions after which you have to buy an upgrade, I just export the wav file(s) and use MC's Lame converter (it's better anyway) to make mp3s.
I have converted a couple of thousand LPs and around 300 cassettes (all made on top of the line JVC and Nakamichi (the Dragon) tape decks). BTW, if your tapes have been sitting for a long time, I recomend that you fast forward through the tape and back before playing them as they tend to tighten up over time and this can affect playback.
Good Luck and feel free to ask questions here, or pm me.