Hello,
What error does it give up when you try to backup this particular user's mailbox? Have you successfully installed the remote agent? Refer to the following technote which might help you with issues regarding XP SP 2:
Title:Backup or restore operations may fail with "0xa000848c - Unable to attach to one of the drives." and "Unable to attach to \\SERVER" after Service Pack 2 has been applied to a Windows XP system
http://support.veritas.com/docs/269701
Hope the above information helps. In case the isue persists, do write back to the forum. However if we do not receive your update within two business days, this post would be marked ‘assumed answered’ and would be moved to ‘answered questions’ pool.
Regards.