I'm quite happy with iShowU for OS X screencasting (screen video capture), but the XP equivalents I've seen have been quite expensive or poor quality. I've got a new one to try for my employer projects (XP): BB FlashBack Express for $30 with an excellent trial policy. It looks quite promising, I'll update this post with what I learn.
Update 8/30/07: That was quick. A complete flop. It died without a notice after the first trivial video capture and left a hung process I had to kill manually. I suspect it doesn't like dual monitor configuration. At least it didn't waste much time. I'll try doing XP video captures using an old digital camera from my closest, and if that doesn't work I'll run Windows remote desktop from a Mac and use iShowU to capture the XP video.