can only uninstall programs that are listed in the Add-Remove Programs window, so if it's not there already, it won't show up in the program itself. Many times the programs listed in the Add-Remove Programs list uses an uninstall.log in order to normally click an item and choose Remove....however, some programs put that log stupidly in the TEMP folder where it places all of its setup.exe files during an installation and as soon as you reboot the computer, those files, including the uninstall.log file, are removed by the program's installation commands. Then you get stuck with an error when trying to uninstall the program later that tells you it can't find the uninstall.log and the program can't uninstall itself. That's where the MS Cleanup tool comes to the rescue.
As for uninstalling one of the XP versions....if they are on two separate partitions, you can just go to Adminstrator Tools/Disk Management, find the drive that has the version you want to get rid of, and format the drive (as long as it's not the bootdrive).