You want to dualboot XP and Mint on a single USB drive? I usually just use GParted for partition management and a XP install CD to fix broken Windows bootloaders and run chkdsk. I never install Windows from disk anymore since I have an fresh copy that I just clone back from.
I've used a bootable XP fix-it CD before, but I won't tell you its name here. I think you would want to mix a USB with that and a live Mint session too. That should cover 99% of your PC needs.
Running on a 4-slot toaster @ 60Hz