Xp activation key limited

Hi Girls and Guys;
Don't know to much about computers, so I hope your all patient.
Anyway I've had this computer about 12 months now and a nine year old boy that tends to muck things up with it. So I'm forever formatting and reloading my windows xp service pack 2 disk that came with it.(approximately 50 times). Well this week when I went to enter the product key it told me that (According to our records the number of times you can activate windows with this product key has been exceeded. Enter different key - Retry).
Well you can't just make one up and they don't give you a spare and nobody told me I had a limit of installations. So how do I get the blasted thing to except my product key so that I can register for downloads that will suddenly stop in 28 days time.
By the way untill now my reformatting has worked fine until two days ago and the bloke on the windows microsoft help line just laughed.

 

I think I'm navigating wrong.
But thankyou for the reply's.
Have tried all the appropriate ones other than buying a new disk and I just woudn't know how to do the thingy with the 2nd drive.