XP Upgrade Version used to Repair?

Hi, i've got an error message on my home built computer, and I no longer have the XP install disk that I need to access the repair console so I can fix the error (Unmountable_Boot_Drive). I have an unopened upgrade copy of Windows XP that I want to return but would use if it could still be used to access the repair console.

As my copy of XP doesn't have System Pack 2, I was wondering if I could use it to just upgrade my old, not updated XP and go from there as well. If not, I'll have to shell out an extra $100 on OEM Windows so I wanted to make sure that it was impossible to use Upgrade XP on another install of XP. Any help would be greatly appreciated.