I'm assuming that ME is your first OS, and 2k was added afterwards.
You really no longer need either of those OS's if you plan on running XP.
What I would do is move all your data (stuff you want to save) to a seperate, non OS partition. Hopefully you have enough space to set one up, or you have an exsisting one. A partition without an OS on it is always a good idea.
If you don't have that/can't make one, I would suggest moving all the data to the 2k partition.
Basically, you want to make sure that your ME partition is clean of anything you don't want to lose.
Boot up with the XP CD and reformat the ME partition. This will in turn make your 2k partition un-bootable, but it will not erase any of the data. Have setup install XP on the partition you jusy formated.
When you get XP set up, go into my computer, and go to your old 2k Partition. Pull over all the data you saved there. Make it clean of and data you dont want to lose.
Reformat your 2k partition and leave it as a data drive.
Make sure to use the right File systems so each partition can see the other.
Enjoy.