Hello Dave,
Welcome to Vista Forums.
There are probably other ways, but Ecsson's method is probably the easiest. If you have been using the Vista install for awhile and have files you want to save from there, you could install Vista on the XP partition (I recommend a clean install). Move the files from the old Vista to the new and then delete the second partition. Personally I'd keep the second partition to put all my personal files on. That way if you ever need to reinstall your OS, your personal files won't be erased. After deleting (formatting) the partition with the old Vista on it, you may still have the dual boot options at bootup. If so you can type 'msconfig' (no quotes) in the start menu search bar, press enter and on the boot tab select the old Vista (the new one should be the default now) and delete it there.
Hope this helps.
Gary