You can manually add the option, by editing the boot.ini file on (typically) your C drive. You'll have to open Explorer, go to "Tools" -> "Folder Options" -> "View" Tab -> Tick "Show hidden files and folders" and uncheck "Hide protected operating system files".
Once you have that done, navigate to your C drive, and make sure boot.ini isn't read only (it is by default). You can do this by right clicking the file, and going to properties and ensure the "read only" box is unchecked.
After that, you'll see sometime similar to this:
;Warning: Boot.ini is used on Windows XP and earlier operating systems.
;Warning: Use BCDEDIT.exe to modify Windows Vista boot options.
multi(0)disk(0)rdisk(0)partition(1)WINDOWS="Microsoft Windows XP Professional" /PAE /NOEXECUTE=OPTIN /FASTDETECT /NOEXECUTE=OPTOUT
Depending on the "partition(x)" section, you will need to duplicate that line and replicate it with the appropriate partition number for your old XP install. In your case, it will probably be the existing number minus one.
Once you add another entry to the list, save the file, and I think you can optionally make the file "read only" again.