'PC' is an increasingly meaningless term in this context these days anyway, given that a significant number of desktops (by which I mean "not servers or embedded systems", rather than something that sits on a desk) run a Linux OS, and Macs are using basically the same hardware. You can't reasonably assume it's running Windows XP any more.
ObTip (not Windows specific, but gets the most use on Windows boxen): A modern computer can be forced to power off by holding down the power button for about 4 seconds, rather than faffing about unplugging the mains or removing batteries. This is a low-level hardware function and should work regardless of what the OS (if any) is or isn't doing.