Camtasia for free (Discontinued but Replaced!) (Updated 2X)
2nd Update (2/7/09):
Techsmith has released another free product for desktop recording called Jing. However, it is limited to about five minute max time. So you'll need to break up your segments. That said Jing makes it super simple to get your video up on the web.
UPDATE: TechSmith has discontinued this offer. If you haven't already gotten your freebie yet you are out of luck. Sorry!
TechSmith has started offering their excellent Camtasia Studio v3 software for free. You use the software to create small but crystal clear movies of your desktop. It uses a special codec, but will squirt out an EXE that will run the codec in a built-in player. So even those folks that have their movie player settings hopelessly ensnarled can watch your video.
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The software is great for teaching and unattended demonstration. If you are running windows, you should consider grabbing it.