i have 2 disks each one having 1 bootable partition. For now both have XP but i plan to install linux (Mandriva) on the 2nd once i get an answer for this problem:

[From now on the term "2nd XP" could be read as "Linux" as i think the problem is the same]

i share my directories under XP using owner/group/permissions (similar to Unix) rather than using simple file sharing i.e. i can create a directory and let my wife read it only and my son having no access on it while i (admin) have full access. Everything is well until i boot from the 2nd disk (another XP with different config). Even if i'm still admin on the 2nd XP, i can no more access previous directories with constraint access (set up when i booted on 1st XP).

I can understand the problem since permissions are PART of the FILE SYSTEM rather than the OS. Thus, since an admin on the 1st installed XP have not the same identity as the admin on the 2nd installed XP, he can't access those constraint directories.
And i suppose i'll have the same problem if i install a Linux on my second disk instead of another XP. And again i'll have the same problem when i'll install vista in order to have it along XP on the same machine. Or for any two Operating systems willing to share directories with constraint access on the same machine. In other words this is a general issue but it got to have a workaround since it's very similar to the permission problem in a network:
when you're admin on machine A, you're not necessarily admin on the machine B. So if you want to have the same identity across a network, something must be done on every machine.

Is it a way to apply the same principle (as on a network) or another solution in order for a person to be recognized as admin on two systems installed on the same machine (2 XP, 1XP and 1 Linux or 1 XP and 1 Vista, ...) ?

I recognize that usually you don't encounter this problem if you use simple file sharing but i need to use file sharing based on user credentials so i need a solution in my case.