I haven’t personally tried it (though I plan on it) but
CamStudio is supposed to be the best Free WinXP one.
One the sound, my thoughts and I am not sure how this would work but is to record the SC and then record the audio after in
Audacity while watching the SC so that you can control the audio.
As a former motion picture sound recordist and radio broadcaster that is worse than the mouse-like, quiet, "umm" filled dialogue that accompanies most screencasts.
Hope this helps.
Cheers,
Jay