To be honest, you made far too many changes, but I expect you now realise that.
When you installed Ubuntu originally, it wrote to the MBR (Master Boot Record) on your XP drive. BUT, this is only a minimal piece of code and it pointed to the GRUB files on your Ubuntu drive. When you installed PCLinuxOS over Ubuntu, it destroyed the links between your XP drives MBR and GRUB.
There are a number of options you could try.
You could reinstall PCLinuxOS, BUT, allow it to write to your XP drives MBR. All being well, that should pick up your XP installation and you'll be back to square one, except you'll have PCLinuxOS rather than Ubuntu.
Or, you could run your XP installation disk and run fixmbr - do a search to find out more. That should recover your XP only installation. To be honest, in my experience it can be a bit hit and miss and personally, I'd only use that as a last resort.
Or, you could find a free third part boot manager and load that on to your XP drive and see if that would work. A decent boot manager should find all your current installations and set them up accordingly. GAG springs to mind, click here as it's well regarded. I've used Boot Magic in the past, but as far as I know, it's not free.