Hi:
Your PC originally came with Microsoft Windows 7 Home Premium (64-bit).
HP Pavilion p7-1234 Desktop PC Product Specifications | HP® Customer Support
HP did not support your PC for any other operating system but Windows 7 64 bit and W8 64 bit, but only supplied W7 recovery media.
If you want to install W7 on there, you can purchase W7 recovery media from this non-HP vendor at the link below.
Computer Surgeons - Windows 7 64 Bit Recovery Kit C4G69AV For HP Pavilion Desktop PC Model Number p7...
If there is a Windows 7 product key on your PC's case, let me know and I will give you a way to install W7 for free.
If you want to reinstall Windows XP, I'm afraid you are on your own.
You would have to buy XP installation media and a product key on eBay or someplace similar.