I dont have Windows CD?

Hello,
I bought a used HP pc 5 months before with window XP pro preinstalled. Now I want ro do a clean window installation, but the problem is the windows CD did not come with pc but it has the products key stiker on it. I went to pc repair shop where I bought the pc and asked for a CD or iso image of the OEM, but they didn’t give me they said bring your pc here and pay $40 we will install windows agian which is so stupied. Can some tell me where can I get the CD? None of my friend has this CD to just borrow it and use my own product key.

Regards