Define "old generic desktop".
If it's a prebuilt computer like the Dell, then chances are Windows XP is locked to that motherboard and won't allow you to boot on the Dell. That's ignoring the 'old' part as it's possible it may not even have a compatible interface.
A new Dell PC should have its own OS, so the question seems a bit odd unless you're specifically trying to run software no longer available for newer versions of Windows.