If you do happen to have a pirated version of Windows XP, then the worst that can happen is that the OS be de-activated. Something like user accounts will stop loading correctly, i forgotten exactly.
Anyways, this is hardly likely...as...I have heard from some of...my...friends
What is more likely is that when you try and update a pirated version of Windows XP through Windows Update, then you'll be denied permission from upgrading elements.
, especially SP2. I understand there to be one common Windows XP Professional CD key that is commonly used on the OS black market
, just as Microsoft have found. What microsoft have done is listed all commonly pirated CD keys and made sure that they are all thoroughly checked when a pirated machine connects to the update server.