Do you have WindowsXP sticker on the bottom of your laptop with the Product Key? If so, then this should be a valid copy of XP unless it's been loaded with another one.
You can only load XP 5 times before the key becomes invalid and, as Gary said, the windows validation check kicks in now when you try to update and says it's not valid anymore. If you're sure it is a real version, then ring the number given and give them the product key. I've done this when I've had to reload XP several times because of viruses/hardware failure and they are happy to comply. Either that or they'll tell you it's not a real copy, and then you can get a new product key for a charge of about £98.
The only drawback of not getting it validated is that with the validation check, you won't be able to get any security/program updates
Andy, February 2007