Im guessing (90% certain):
Edit your boot.ini file.. delete the line about "cancel Windows XP setup"..
That should do the trick.
For example, my XP boot.ini file says:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Microsoft Windows XP Professional" /fastdetect
The boot.ini file sits on your XP system partition. Ensure you've unticked "Hide Protected Operatin System Files" under your folder options. Otherwise, you won't be able to see the file.
I'm not familiar with ME's architecture, but I'm guessing it's similar to Win98.. it doesn't use the WinNT structure. Therefore, there is no need for a boot.ini file.. Check if you have one, if so, delete it (back it up to floppy in case).
A word of advice: NEVER upgrade from one Operating System to another.. always do a fresh install.
My opinion: XP sucks.. but hey, I still use it. Win2K is the best.