I assume you're using XP's own dual boot and not some third party software.
Boot into XP first. Insert a floppy. In My Computer, right click Drive A and select Format. On the small pop up window, click Start (and do NOT tick any of the option boxes). You must take this step in XP (not ME), and must do so even with a brand new floppy.
After formatting, go to your C drive. Find the 4 files ntldr, ntdetect.com, boot.ini and bootsect.dos. Copy them to the floppy. [If you see the file C:\ntbootdd.sys as well, copy that too, but it is very unlikely you will have that file.]
If you don't see them under C, click Tools, Folder Options, View tab, (1) tick Show hidden files and folders (2) untick Hide file extensions for known file types (3) untick Hide protected operating system files (4) click OK. You should see the files now, and copy them to floppy.
Remove floppy and label it "ME/XP dual boot floppy", or whatever you like to identify it. This floppy not only serves as a backup of your dual boot files, but also can boot up the computer (bypassing the hard disk) and give you your usual OS selection menu on bootup.
Then format C and reinstall ME. After booting into the freshly installed ME, copy the 4 files from the floppy back to the C drive.
This restores the dual boot files to the C drive, but an extra step is required. After reinstalling ME, the boot sector of the C drive becomes a ME boot sector, and when you start the computer, it will only look for the ME boot files, not the XP boot files even though they are there.
To re-create the XP boot sector, insert XP CD, and restart computer (making sure that in BIOS CDROM drive comes before hard disk in the boot sequence).
After computer boots up from XP CD, and you get to XP Setup screen, go into Recovery Console by pressing R. It will then scan your hard disks for XP installations, and show it as 1: X:\WINDOWS
where X is the drive letter for your XP partition.
Press 1 and press Enter. Then enter your XP Administrator password.
At the X:\WINDOWS> prompt, type
and press Enter. Confirm the action. The fixboot command should rewrite the boot sector on C so that it becomes an XP boot sector.
Then back at the prompt, just type exit and press Enter. Computer will restart. Remove CD and let computer boot from the hard disk.
You should get the boot menu again, and make your selection.
If you don't get the boot menu, and computer goes into ME automatically, post back.
In the meantime, if things don't work out, you can boot up computer with floppy and go into XP via the floppy.