I'm not sure going from XP Home to Xp Pro is an upgrade. Why the need for Xp Pro? I would buy the cheaper XP Pro OEM and do a new install of that OS. Of course back up all important data to removable drive or disk first. Also, the system drivers would have to be installed from the system/MB Cd or can be downloaded from the system/Mb online product page. Another route may be to use an OEM installation disk and do a Repair install. That would leave all the personal data/apps in place and replace only the Windows OS files. Not entirely sure if this route would get you to an XP Pro installation, but I believe it would.
List the system specifications including brand and model number if it applies.
You can only purchase OEM software if you bundle the it with hardware. Stores are not allowed to sell you OEM without hardware to go with it, so you won't be able to just walk in and buy just a copy of XP Pro OEM. If you can, that store is actually in violation. I think retail copies of XP Pro will be pretty hard to come by now.
License agreement aside, you are able to keep your OS and settings going from Home to Pro. I've done it many times and you perform what's called an in-place upgrade.
Stores are not allowed to sell you OEM without hardware to go with it, so you won't be able to just walk in and buy just a copy of XP Pro OEM. If you can, that store is actually in violation.
Of course you can buy an OEM copy of a microsoft OS at any store that carries one. You can purchase a tube of artic silver or a bag of zip ties for $1 and that is considered hardware around here. The stores certainly do not have the OEM microsoft sitting in thier inventory with the idea they will not sell it to you. The stores even advertise OEM Windows OS for sale. Anyway.