Windows XP reinstallation

I have a 10-year-old HP 6630 desktop that originally ran Windows 98 but was upgraded to XP Home a few months after XP was released. It ran absolutely fine on the computer (500MHZ Celeron, maxed out at 256MB RAM, a 9.5GB HDD [after formatting], CD drive) for a few years, but then it began to run REALLY slow just a few years back. I've had a few other computers since that one, and my dad's ready to bring it to get recycled since it's hogging up space in our laundry room and nobody's used it for a few years. He said I could do whatever I want to with it otherwise, so I figured I might as well try to fix it up a bit for the heck of it and to serve as a learning experience. It had a 40GB hard drive in it for a little while, so I moved some programs and files over to it, but XP still thinks the programs are still there and won't let me get them out of the OS even though the other HDD was removed a while ago. It also won't allow me to uninstall old programs. This computer used to run perfectly fine with XP, so I think I just need to reinstall Windows. My parents never keep boxes or discs with their computers unfortunately, so the system restore discs are long gone. I do have the upgrade disc and product key for XP though, so I want to try to reinstall it. At some point, service pack 2 was installed. And when I enter the XP upgrade disc into the computer, it won't let me install/reinstall it since a higher OS is already on the computer. I really wish I could wipe the HDD and just put Windows on it, but this the only OS disc I have compatible with this computer is only that XP upgrade disc. What can I do to wipe all programs and settings off of this computer and reinstall XP Home (service pack 1) to use this as a simple internet computer? I can reinstall a few of the programs (like Office XP) I need to on it. Thanks in advance!