@Mods: a very Interesting discussion here on recording from Cassettes, we better move this post to a relevant section and continue it there.
Recording through an onboard sound card, that too on a laptop is the first stumbling block. The onboard components are not necessarily well packed to avoid jitter and circuit noise. This added with inferior ADCs make recoding quality suffer.
I'd recommend you get a decent pro USB soundcard, and try the same. Also consider persiflage's suggestions.
I have been using Audacity for the same purpose and it does a decent job, polderbits is also a good tool, however it may get a lil intimidating for starters.
Postprocessing is almost always necessary while recording from analog sources, u'll neeed to master with trial and error the regular NR techniques with the editing tool you use (Audacity is open source and free).
Let's get other expert's comments on this, Chaosdada you reading?