I don't think there are any major reasons to go over what has already been said in this thread.
At the end of the day it is totally up to each individual if they want to use pirated/illegal software on their machines or not.
"I'm not the boss of you" - however from a legal point of view then only legal options can be discussed on the forums.
If you like it or not, if you own an OEM license and you replace the motherboard (for any reason other than a hardware failure/warranty reasons) and then use the same OEM license to install XP or Vista you do so illegally.
You are no more license legal than the person who downloads a copy from P2P.
I know people don't like to hear this and I've seen many a messenger shot for pointing this out, but it is the way it is.
Remember you agreed to the license before you installed the product.
Ignorance is no defence, so saying that you were not aware before the purchase is not a valid defence.
Nobody buys anything without first making sure their purchase meets their requirements.
I wouldn't buy a PS3 game and then complain because I hadn't got a PS3 to play it on - I'd check what I needed before making my purchase.
What can be done legally and physically are two entirely different things.
If I wanted to, I bet I could take a single OEM copy of Vista, install it on 5 machines and get each of those 5 machines activated.
This is physically what I could do, legally it is only a valid license on the very first machine I installed it on and then only to the point where I replace the motherboard in the future, when I then need a new license.
If you want to move the license from machine to machine then purchase retail.
You can read the basic guide to the licenses here:
http://www.cifi.com/computing/vistaoemretail1.html
This should cover most of the issues on what you can and cannot do within the restrictions of the various licenses available to you.
Just be aware that by replacing your motherboard and then getting MS to reactivate the OEM license you are using the product outside of the license agreement and what you are doing is no more legal than any other thief downloading a copy from the net - despite how you feel about the rule or how you feel because you did buy a copy at some point.