A nice way to keep these systems available (while not having to physically maintain them) is to put them in a virtual sandbox using a tool like kvm. While kvm makes it relatively straightforward to install WinXP from a CD (as long as you have the proper licensing key), it is more challenging to transition a pre-existing hardware windows XP installation into a virtual instance, due to Windows only wanting to boot to ide chipsets that it remembers being installed to.
In particular, booting a disk image pulled from a soon-to-be-discarded physical disk can produce a Blue Screen of Death (BSOD) with the message:
Stop error code 0x0000007Bor
This seems like it's roughly the equivalent (in a standard debian GNU/Linux environment) of specifying MODULES=dep in /etc/initramfs-tools/initramfs.conf, and then trying to swap out all the hardware.
At first blush, Microsoft's knowledge base suggests doing an in-place upgrade or full repartition and reinstall, which are both fairly drastic measures -- you might as well just start from scratch, which is exactly what you don't want to have to do for a nursed-along legacy system which no one who originally set it up is even with the organization any more.
Fortunately, a bit more digging in the Knowledge Base turned up an unsupported set of steps that appears to be the equivalent of setting MODULES=most (at least for the IDE chipsets). Running this on the old hardware before imaging the disk worked for me, though i did need to re-validate Windows XP after the reboot by typing in the long magic code again. i guess they're keying it to the hardware, which clearly changed in this instance.
Such silliness to spend time working around, really, when i'd rather be spending my time working on free software. :/