I'm going to leave this post; I know, we're against piracy, so why leave this?
Well, WGA has hit a lot of LEGIT/OEM installs, including my own... I own several WXP Home and WXP Pro licenses, from PC manufacturers (Dell, HP, Compaq, etc) and had 2 of them listed as invalid via WGA...
And I've heard of this happening all over the internet... What does M$ have to say? "The system is not perfect"...