What is the model number of the HP?
At one time HP had a install disk for Windows XP. that OEM version of XP did notask for the product key if you were doing the install on a HP machine that was licensed for use with Windows XP. If you HP mchine is one of those models, and if your can find the origianl HP OEM Intall for XP, then you can do a fresh install and not have to uhe product key.
I had a couple of HP macines at one time and I still have the HP OEM CD. Sorry, I am going to keep it. I won;t let you have it. Howeverm you might check with a local computer club or maybe a dealer who has been in the PC service business for a number of years.
Look atthis:
That is not a HP, it is a Dell. He wants to sell it for about $21. Don't buy it, it does not have the drivers your need for the Hp.
Here is an Hp, but it is SP2
You might find some place that provides the SP3 download. I am not sure about that.
I believe taht CD withh work on a HP made from about 2004 through 2007. I think.