Write access to Program Files folders

I have three XP Pro machines connected as a workgroup (i.e. no domain
controller). I need an administrator to be able to copy files from any one
computer to the Program Fils folders of any other (as well as to be able to
access all other folders).

So far as I can tell the configuration of all 3 computers is the same (and
Program Files folder permissions the same), but although I can copy files
successfully between two of the machines, I cannot do so to the third. I get
an 'access denied' error.

What do I have to do to make all 3 machines fully accessible to an

NB: The administrator has an identical account and password on each machine.

John Franklin