Camtasia Studio works very well. Unfortunately it isn't free though, but they give you a 30-day trial to tray it out.
In my opinion, it outweights
Fraps (which has a free edition) by far.
It depends what you are going to record, I suppose. But I'd say go with Camtasia if you want real good capture quality (it will also let you record only a certain area of the screen and has a nifty pause function that will reset the cursor and everything to what it previously was when restarting recording. So you can look up stuff and not make a fool out of yourself if recording a tutorial or whatever

).