Do you have a IMAGE from your XP system made by a drive imaging program on that external or a direct copy of the drive/s in that XP system on the external, IE a Mirror Image.
If it is a direct COPY, MIRROR, of the XP system you can place that into a folder and then go to the security tab in the properties of that main folder and set permissions to not allow anything but List folder contents and Read for every user listed in Groups or user names.
But then you might lose your own rights to that folder.
What I suggest is to get a Drive imaging program, there are some FREE versions listed in the tutorials on this forum, create a image of that system, which will store all the data in one file, then you can't make any changes to it without first starting that imaging program and Mounting that image as a drive. And that way you will always have a backup (Snap Shot) of that system.
If you are using a direct copy you can't restore that system with it. There are things that need to be placed in specific areas of the hard drive, mainly the MBR, to allow any system to boot to the OS. Just copying all the folders, files and whatever back over to a hard drive does not put those things in the correct places.