Not sure how to power them back on, but there a couple way to power off... you can build a program that accepts a network connection from the server, and behaves accordingly... probably not the most efficient method.... the best method, in my opinion, is the shutdown command, available with XP. Here is a link: http://www.computerhope.com/shutdown.htm.

So it would look something like: shutdown -i

you could put this in a task scheduler, or build a program that checks the time, and runs the shutdown command at the given time. As for powering on..... I'll let you know if I find anything.