I thought I’d mention this in case it helps anyone else.
If the copy of Windows XP originally came preinstalled on a PC, it probably won’t use the standard activation system. (And unlike later versions of Windows, it will not boot to the desktop at all!) Instead it was ‘preactivated’ by a DMI string in the BIOS & matching OEMBIOS files on the drive (SLP 1.0). Originally, if for example the motherboard had to be changed, you could use the license number on the Windows COA sticker to reactivate the conventional way. But this was disabled by Microsoft c.2005. There were third party tools which could easily add the DMI string to most BIOSes, but these would not work for a VM.
Fortunately it is possible to use an image taken from a SLP 1.0 OEM installation of Windows in VMware or Virtualbox. (Although doing so will violate the license agreement, which is tied to the original hardware. But if one is just trying to migrate to a newer machine…)
For VMware, there are modified BIOS files available. For VirtualBox (IIRC) it was simply a case of modifying the VM configuration text file to add the correct DMI string for your machine.