Most likely is that the permission on this directory have become "corrupted". It is possible for even an administrator to be denied access to files/directories. When this happens, you need double click "My Computer", then "C drive" then "Documents And Settings", etc until you get as close as you can to the directory in question.
Right click on the directory you cannot access, and select "Properties", then click on the "Security" tab. Then click on "Advanced" at the bottom of this. Then click on the "Owner" tab. Change owner to "Administrators", and tick the box to replace owner on subcontainers and objects. Then click OK.
You will now have access to change the permission for this directory (in case you did not previously). You can now add "Administrators" to have "Full Control", which should at least give Administrators access to this directory now.
I know it sounds long winded and complicated, but that's Windows XP for you!