I have machines with OEM XP installed that have been here since XP was first available. They have been kept up to date with service packs and Windows updates and they are still going OK. I also have machines that have OEM XP with included SP2 on the disc. I don't notice any difference in performance between them (although there is a difference in performance of the machines they are installed in).
As you have been doing a lot of changes to your machine, perhaps it's time for a clean re-install of the operating system and start afresh. I would doubt that a new version is required. Just ensure that you install SP2 and take all precautions about infections before connecting to the Internet.