No offense, but "'WinXPProOEM.com" should have been a fairly sure sign that something dodgy was going on, they're not claiming that it "was legal and now it's not", they're claiming that it was illegal in the first place, which sounds pretty likely to me. I haven't heard of anyone who bought from a genuinely legitimate place now being marked as pirated. It's not even as if anything's been disabled - it's just letting you know that you have an illegal copy. You're exactly who Microsoft are trying to target - those who bought the software without knowing it was illegal. If you have your receipt then you are eligible for a free copy, if not then a discounted copy, Microsoft are being nice rather than "evil" about this.