Was it a different model? I don't know that there's even an interface to pull something like a serial number for a machine's motherboard, which would make it impossible to generate a hardware ID based off that kind of information. I'll have to play around with Everest later ...
Since Windows XP, Microsoft has used a variety of information to generate UIDs for activation purposes, including CPU IDs, CPU model, memory models, and so on. I don't think a same-model motherboard is a distinguishable, even for them, though.
Edit: D'oh. Forgot that the BIOS probably has a serial burned in.