Back in Mark's first post we had:

"Attempts to restore it to another drive, trigger XP's copy protection.
XP runs a checksum on the serial numbers of over a dozen bits of H/W on
your system. (eg. harddrive, CPU, video card, any number of things you
might want to eventually upgrade, or might break from moving parts.)"

So it is clearly not an MBR problem or anything like that.

Like you I am puzzled by the problem that Mark's friend faced.