If you save the old system files [and you did not convert to ntfs] then you should be able to uninstall xp from add / remove programs in control panel.
Now upgrades from win9x to nt based os do not generally work well. I would backup anything you want to save and do a clean install of xp; not an upgrade. You will have a more stable, smoother running system. If you have an upgrade disk and not a full version, you can still do a clean install of xp. During the install, setup will ask for your qualifying product. You will insert your me cd and setup will check it and continue. Note it will not work with a recovery disk that comes with most store bought computers; it must be a full version of ME.