Weaklink -
I believe XP also returns 'Windows_NT'. As for running across the network, sure, it's fine for NT-class. However, I would recommend you structure your script to run from any DC via the netlogon share, versus a single server. If the one server is off line no users will run your script. This variable is %logonserver% for NT/2K systems. Since XP essentially 2K from networking perspective, should be same for it as well.
Bill