if you want to edit the boot.ini to dual boot both versions, I assume you have both versions still installed.
you probably have something like
code:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /noexecute=optin
where partition(1) might be something different
to add a boot option, you should only have to add a copy of whatever line you have in YOUR boot.ini and change the partition number and OS name
/ 
