A good way I recently discovered after trying many software capture tools with slightly disappointing results (probably due to my low spec PC) was to use my wife's laptop with a TV out via a s-video (it was a Dell Inspiron 1525, a lot of Dell laptops seem to have this facility). I routed this to a DVD recorder and the sound via the headphone out and recorded a webcast this way with really good results. I then authored a DVD with menu etc using the recording from the DVD recorder
Clearly its better if you can capture the stream direct, but in some cases (such as the one I was after from
www.channelbee.com) this wasn't an option due to the proprietary player and DRM issues.