There's a shutdown command I ran into yesterday and didn't think much of it. Do a search and first figure out how to shutdown a Win machine from the command prompt. Once you have that working, then you're ready to interface it with the web.
Interfacing to the web will be the fun part. I can understand if you're referring to computers on a local intranet. You won't be able to do anything for computers out on the regular Internet.
As an administrator, I would think you might be able to push commands onto another local machine and if you can get this to work, you'd have the web server push the shutdown command(s) to the computers you decide.
If that doesn't work, then there's always the old school way of doing it which is have all the local machines periodically check a local network drive and if a file (of your choosing) exists, the computer shuts down (this requires some programming and scheduling to reside on each machine).
A little over kill, but you could put PHP on these XP machines and setup a socket server which listens for a shutdown sequence (you'll want to have some authentication take place). The web server would then connect to each of these machines and issue the shutdown sequence/authentication.
Just a couple ways of doing it...
