Can microsoft legally do that? Forgive me for asking and being dunce about it as I really don't look into or read anything at all about microsoft's crap but is it legal for them to make you purchase another key because you decided to buy another motherboard? I mean it's like calling up Ford and asking permission to start my car after changing the brakes in it. If that is the case we need to start looking more into linux quickly.
The reason why you pay more for the Full (Retail) version of Windows versus the OEM version is that the license agreement is different. (remember that long, undecipherable thing you clicked through?) It is a contract, just as legally binding as if you had sat down in a lawyer's office, written an agreement then signed it with witnesses. With OEM version, especially that sold through one of the big manufacturers, you have agreed to keep Windows on that one computer, the Retail version has a more liberal agreement between you and MS.
U.S. contract law allows for a contract to be valid even if it favors one side a whole lot more than the other, and MS has written a contract that is grossly unfavorable to you but still legal, though to be fair other software makers have written worse.
Normally a contract must be signed in ink by both parties, this is what's called a "Meeting of the Minds", and is one of the seven elements of a binding contract. However through various court cases the judges have allowed "Shrink Wrap" or "Click Through" agreements to have the same legal force as an inked contract with witnesses. This is what is called in the Legal community as "Case Law", it's a phenomenon in the courts where a judge essentially creates a new law allowing "Click Through" agreements simply by ruling that they are legally binding. This can happen when the opposing side presents a weak case and does not file proper appeals. Other lawyers arguing similar cases will cite the original case where "Click Through" was allowed thereby forcing the next judge to make a similar ruling, as this snowballs the original ruling gains strength.
These sorts of shenanigans were allowed in the early days of computing when there was a very large fear in this country that foreign countries like Japan would overtake the early lead that the U.S. had in computers and lock us out, just as they had with consumer electronics. If you doubt this then consider that Intel invented the RAM memory chip and now most if not all memory chips are made in Asia, Intel was forced out of an industry it created! It came close to driving them out of business, they were only saved by moving in to what had until then been a sideline of theirs, the CPU!
There was a very legitimate fear that all computers would one day be Made in Japan. So the Government and the Judicial system gave the computer industry wide leeway to do all sorts of things that would normally be of questionable legality if not outright illegal. As time passed it started to cause problems for everyone involved, people like you and me are stuck with not being able to do as we please with an operating system we paid for in full. And even the computer industry itself is hurt. Every few months one tech company has to settle with another over patent infringement. The patenting of every minor step, process or concept in the making of tech is routinely used to hamstring competition. I do believe Amazon.com has a lock on "One click checkout" in internet commerce. The tech industry generates ENOURMOUS amounts of litigation.
So, yes, it is legal for Microsoft to make you buy a new key for your OS, if you swap out enough parts then they consider you to have built an entirely different computer, which requires an entirely new key and full payment for that key. Because of the lopsided contract you 'signed' what Microsoft says you have to do is legal.