Definitely not legal (as in "licensed by the books" - the real legality of all this is not well established), though it's moral enough. The licensing for corporate versions are quite explicit that they are to be used only by authorized Volume Licensing Partners (or whatever the term du jour is).
That being said, I can't imagine you could get in too much trouble. As an individual puchaser, it's unlikely MS would ever find out, and if they did, and you produced a valid retail/OEM license, they would certainly not waste money and risk bad publicity prosecuting you. At the worst, you'd just have to transfer over your settings to a new installation. Of course, rumors occasionally surface that the widely copied corporate editions will be locked out of Windows Update, but we'll see...
But, honestly, it's just not that hard to activate a legit copy. The MS people on this won't ask questions about your date of purchase, computer hardware, sexual orientation, or anything else you don't want to reveal. Last time I reactivated, it took less than two minutes, including time on hold. Not worth the trouble of worrying about the legalities of the alternatives.