I doubt that the HP OEM Windows XP disk will even install on your new system.
But you can order a "system builder" or "OEM" version, from places like Newegg. It is the full version, and is even less than the upgrade versions.
But once again, this version can only be installed on 1 computer. If you ever upgrade again, you cannot install it on another system, so it's up to you. Pay now, or pay later. A lot of people figure what the heck, by the time I build new, there will be another version of Windows, so I will just buy another one. But if you do a lot of upgrading, and you reinstall Windows often it may be cheaper in the long run to buy the full retail version.
So, it's up to you. Save some money and buy a OEM/System Builders version, or pay for the full retail version. Be aware that for what ever it is worth, the OEM/System Builders version is sold as is, with no support from Microsoft. So if you ever need to call Microsoft for anything, you are out of luck. I don't think that actually means anything to most people, just so you know that before you decide.http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754&cm_re=windows_7_oem-_-32-116-754-_-Product