I think a different motherboard qualifies as putting Windows on a "different PC." You may have better luck on Hardware & Devices - Windows 7 Forums
where the system builders hang out. I'm sure some of them have changed motherboards in the past. Maybe it's just a bad guess on my part. I don't know because I haven't changed motherboards. But it seems to me I've read some stuff in the past that it will accept a different HD but a different motherboard it figures you're trying to put an old Windows onto a new PC. I'd ask on that sub forum.
So as not to get flagged for multiple post with the same topic I recommend deleting your post and posting the question on that group. If it won't let you do it with the Edit button try contacting the moderator(or just ask him to move it for you.)