The disk lettering is an internal construct of the running system (entries in its registry), and has nothing to do with the boot process.
EasyBCD is a BCD manipulation tool. The BCD is a feature of Vista/W7, it doesn't exist in XP, so you couldn't use EasyBCD anyway.
You can easily set up a multiple XP boot with all XPs seeing themselves as C:\
if they're on separate HDDs.
It's more difficult if they share a disk.
Generally speaking, the rule is
Install an OS from a running system by clicking on setup.exe on the installation disk and the new OS will take the next available letter, unused by the running system. (i.e not C:\)
Install an OS by
booting the Installation CD and it will use C:\ as the OS partition letter.
Note the
generally above though.
I happen to know from experience that XP, installed from the booted CD can still use notC:\ if it detects another partition with Windows installed. (This doesn't seem to be the case with Vista and W7).
You could try Installing the 1st XP, then when it has completely finished, use a partition manager app to turn on the "hide" bit in the partition table, in an attempt to stop the 2nd XP install from detecting the first. (and unhide it again when the 2nd install is finished)
It's not something I've done (I don't mind that my XP is D:\. It's not a problem as long as the running OS cannot
see a C:\ disk which isn't the OS), so I can't guarantee that it works, but I can't see another way of doing what you want.
Why is it important to you that the OS is C:\ ?
The only problem of the OS not being C:\ is that 3rd party apps (Adobe notably) will try to put common files on C:\ even if you instruct the install to put the app on D:\.
If C:\ is a
different OS with it's own apps then you'll get the installs scrambled together with unpredictable results.
This only happens though if the running OS can
see a C:\ drive.
You can use a
registry hack before you install any apps to make C:\ invisible, or you could install grub4dos as your boot manager and use it to hide the other XP each time you boot one.