The KMPlayer is what I use to for screencaps. If you have an LJ, there's a tutorial here (you have to be a member of ~dj_capslock to view it, though) on how to use it, but basically you right click in the program, choose "advanced capture" from the capture modes, and tweak the settings with the frequency with which you want it to capture images. You can also specify the folder they go into, and a prefix for the images (so it'll be xxxx-00001, xxxx-00002, etc).