Upgrades are iffy. You will have to upgrade twice to get the latest version,
10.04 -10.10 - 11.04. The chances of this going off without a hitch are
slim. I would backup your data and do a fresh installation of Ubuntu,
leaving XP alone. Save data there, too, just on case. You can try upgrading,
it is your time and effort. I like starting with a clean slate. If you do
this frequently out may be worth extra time to make a third partition for
home. That way you can reinstall and reuse your home, preserving data and
settings.