First off, if your Home Premium license is OEM, it may not be installed on any other computer evar. If your license is a retail one, then it may be installed on computer after computer after computer, as long as it's on just one at a time, and there's no special uninstall routine that I'm aware of.
Regarding your goal of installing Vista Ultimate upgrade, one option would be to
1) run the Windows Easy Transfer wizard (Start > Search >
transfer) and save your settings, documents, etc to a removable drive or a second drive.
2) manually back up anything that resides outside the scope of the Easy Transfer wizard
3) turn the computer off and unplug: memory-card readers, extra hard drives, external hard drives, USB sticks, etc. This ensures that drive letter
C:\ is available for your boot drive.
4) install XP Pro and then upgrade it to Ultimate. If your XP Pro CD-ROM does not have Service Pack 2, be especially careful to
take precautions against worm attack (scroll halfway down to the precautionary info there). During setup of Vista, watch for the
checkbox for automagic Activation :camera: and uncheck it. Activate after you've got all your drives, hardware and drivers installed.
During Vista setup, make two accounts: one to be your dedicated Administrator account, and the second one to be your
Standard account.
5) After Vista is set up, hook up your other drives, take Ownership and adjust security on their contents as needed (hint: your Users group needs security clearances), and then run the Windows Easy Transfer wizard from your Standard account, to import your settings and files.