Are you asking as a student wanting to record a lecture given by a teacher, or as a teacher wanting to record lectures to present? I think there is a world of difference between the two.
Recording a lecture to present, you could use anything to record the voice, such as garage band, and then overlay it on top of your presentation tool, or even record directly into the presentation software.
If you are recording someone else's lecture... Well, I don't think you are going to get good results out of this. Assuming that there is no visual material to record, and it is just audio, you are still left with a poor microphone in your laptop, and will end up with a bad audio recording. You might be able to get a little better by using an external microphone, but it will either have to be USB, or you will need to use an external audio input device connected via either USB or firewire. I still doubt that it would record well enough to be usable unless it was a small environment.
-jt2