If you do have a legitimate copy of Windows XP on your machine (as in using the serial from the case of your computer) then it's possible someone else with a serial key generator just randomly happened upon your key, You have to call up Microsoft support for this and they can re-enable your key for you.
I haven't had to do this since they stopped supporting XP though so I'm not 100% sure you can anymore but they're pretty helpful and it's a very common issue.
Answers & Comments
If you do have a legitimate copy of Windows XP on your machine (as in using the serial from the case of your computer) then it's possible someone else with a serial key generator just randomly happened upon your key,
You have to call up Microsoft support for this and they can re-enable your key for you.
I haven't had to do this since they stopped supporting XP though so I'm not 100% sure you can anymore but they're pretty helpful and it's a very common issue.
Here's Microsoft's "Contact Us" page