You need a minimum of two partitions
An ME disk
An XP disk

On the the c: or first partition install ME
on the d: or 2nd parititoin install xp

XP will pick up the me install and manage the dual boot for you (unless your name is shamus... )

Put the data that is common to both on a 3rd partititon....

But remember you wil have to install all your software twice, once for ME and again for XP, so make sure that your partitions are big enough to handle the restore files, the swap files and the proggies.

Partittioning this way also means that if you toast an os, it is relatively easy to reinstall.