there are 3 things to know when copying/moving
files on a ntfs partions/volume

1. copy will create a new file and permissions
to that file are inherited from the parent (root or folder )

2. moving files on the same partion only updates the the reference
to the objekt itself and the permissions are unchanged(internally)

3. moving files on a different partion will copy the file to the new
drive, (creating a new file)inheriting permissions set on the parent, and the old file are deleted.

You made a move to the other drive, that is...a new file is created inheriting the parent permissions.

The parent permissions on the other drive is set with the dacl
(discretionary access control list ) that contains the ntfs rights for the users on the vista install, that have different sids ( that is
a uniqe id for every user on the system)
the sid on the vista install does not match the sid on the xp install, and xp cant locate any user with the corresponding sid...givng you a access denied message.
( the dacl contains a header, a sid(user the rights belongs to) and any access control enteties)

There are on simple solution.  On the target drive, explicilty add you account to the parent folder that you want to move files to, and give the account full control. Any subsequent moves to that folder, wil then inherit the parent permissions containing the sid (your user on the xp install)