I have used for about 3 years computers based on the mini-itx standard
www.mini-itx.com can give you an overview of the available boards.
the 400/600mhz boards , with a laptop disk use about 10 watts. This gives the advantage of a window XP platform , meaning you can run anything on it but very low power usage.
Compared to the 2-3 ghz machines , there slow , but for a basic PC or running data-logging , web-serving there good little boxes. This web-site was run on one for two years !
I actually have 2 spare ones now that I would sell cheap ($75) , send me a private message if your interested