Could be. An SMBus controller bug that the BIOS knows about and avoids/works around when it reads the SPD, or possibly one in the Windows XP SMBus driver. I used the XP in-box drivers and not the Toshiba ones, which include a "Common Module" driver so it's possible that this deals with the issue.
But now I've figured out what seems to be happening, albeit at a cost of three SODIMMs, and more importantly how to avoid it happening again, I'll leave it at that as it's a freebie job for a neighbour.