i have always been completely perplexed by all this talk of oem vs retail copies of windows and it being tied to one machine and all that. i have never had this issue and though i have read the links provided that tate there is some difference i can't say i have run across a key that did not migrate to a new machine.
i spend a lot of time fixing and upgrading donated pc's to give away through my church and when we ditched a pc i always add whatever windows key came with the machine. a quick photo and written list of keys is nice so we can use them as needed for other machines. keys from xp, vista, 7 and 8 have been migrated this way and not a one has been rejected on a new machine. these keys come from prebuilt pc's from all brands and are simply the one from the sticker on the case.
wish i knew what the deal is but despite the links provided i guess i have never run across a truly "oem" windows key. i buy the oem versions of windows from newegg and such with new builds and have seen them migrate to other pc's as well.
i'm still confused but i guess i'll just keep on doing what i am doing and let others worry about what version's key may or may not be reusable. what i do not see said though to folk who are asking is this line "give it a shot, if it don't work then buy a new key. but try your old one first just in case"