Use Yawcam, AmCap or Debut Video Capture for this.
All these are free and simple. Yawcam requires Java, however.
EDIT: OK, here is some more info re. above apps.
Yawcam can capture both photos and videos, and can be used for motion detection too.
(capture video only if movement is detected)
I have not used AmCap much, but it is a often used and popular app.
RE Debut Video Capture, it used to be a Trial, but now it has a free version too, so you can use it without paying (for non-commercial purposes). It has more features, for example besides being able to capture webcam photos/video, it can also make video recordings of your Windows desktop/screen.
It does this very well - you can specify the captured video file format, and also capture all or just part of the screen, and also it is great for software demos, as it shows left/right mouse button clicks in the video, plus shows a yellow spot around the mouse pointer.