I was trouble shooting a network problem with one of my XP Pro computers recently. (Which turned out to be my cable modem's fault.) I decided to try another network card in the computer. When I rebooted I got the message that I needed to re-validate windows. Which I did. The change in network cards was the only change that I have made to this computer within the past year.
It happen again this morning when I upgraded the hard drive to a larger one in another computer. This one had XP Home and is the only hardware change to this computer in well over two years.
It seems that a single hardware change will now cause Microsoft to insist on re-validating an XP based computer.