Ouch. I'd not do that since your fallback position is unstable. Without the OEM CD or CD to let Windows XP's UPGRAGE take a look at, you shouldn't be updating this OS yet.
Fix the base issue first.
My guess here is you are not logged in as administrator, but there were some HP's that needed a PATCH to install SP1. Such OSes may need help with SP2, but such is not in your post and since it's possible your machine may not recover from an OS loss, my best advice is to cure the underlying issue first.
After downloading SP2 it will only install approximately 50% of update and then a message box pops up saying "ACCESS DENIED" and it goes into the uninstall mode. I am running XP Pro upgrade from XP Home that came with my pc and do not have the OEM windows disk. Can anyone help me with this problem.