My Documents has some weird permissions set on it (that wouldn't normally be on a file in C:\), designed to restrict access to anyone but the original users.  Permissions for everyone else (including the administrator on the XP drive) look like this:

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You can't do anything with the file, because the account referred to by the SID in its permissions doesn't exist, and no one else can do anything to the file.  However, as an administrator, you can fix it.

On the Security tab of the file or folder you moved's properties sheet, click "Advanced".  Change to the "Owner" tab and change the owner to you.  An administrator can do this regardless of the "Take Ownership" permission, since they are admin and have full control over the machine.  Press OK and if it prompts you to apply it to child objects or subfolders (I forget the exact wording), say yes.

Back on the security tab on the file's properties sheet, click "Add" and type your username in the "Enter object names to select" field.  Give yourself full control, and you should be able to do anything you want with the file (move, copy, delete, edit, open, etc.).

[EDIT] I would advise you to not let chkdsk in Vista run on the XP drive (and vice versa), and especially not let it mess around with SIDs on it...  all the XP SIDs will always be invalid when running under Vista.  Fixing them would be (to say the least) bad.  It might not mutilate anything, but if it did, it could make your system inoperable.