It's a standard XP thing - it needs activating to use it. If you've been using XP for a long time you've probably already activated it (IIRC you get 30 days after first use to activate it). Here's an article on it: Windows XP Product Activation

You shouldn't worry about it (unless you make major changes that might require reactivation - I mostly rebuilt my system a year or so ago and had to telephone to get a new activation. I probably couldn't do that now - I changed the motherboard and processor and I think that counts as a new machine now!).

Mick