If you have the pro, it will install, but maybe it will not install on the file system that ME uses, without formating.
If that is the case, XP home will not install either.
I recall that XP stoped support for some file system. Maybe FAT or FAT32, and that would be your problem right there.
BUT I would never install on an old operating system, I would do a clean isntall on that drive.
That is in the install options, remove the old partition and create an new one. Then format.
This will destroy all data and XP should install fine.
But you have to backup first, because this will destroy all the data, pictures, documents what have you.
Did I mention that you will loose all your data if you do this?
So backup any data you realy need, and then destroy the pratition using the on screen instructions.