Is this a question of is it legal, or does it work?
I Purchased a full version of Ultimate for my Dell M1710 and installed it.
I then loaded my original version of Home Premium onto my desktop, the product activated successfuly there even though it was preactivated on my Laptop.
Previously with XP I have changed mother board and hard disk in an older Desktop. After reinstalling the version of XP the system asked me to reactivate via phone.
When speaking to the operator from Microsoft, they provided a manual code for me to reactivate.
I am not sure legaly how all this works, but In my head as long as I have the original disks complete with the original sticker and original activation key for each PC... Ie 3 PC's 3 individual disks and codes, I sleep soundly in my bed at night!