samba - Jan 2006 - Backing up a windows 2003 server to a samba share

Good evening/morning/afternoon.  This may be a little off topic but I 
was wondering if anyone has had any experience backing up a Windows 2003 
SBS to a samba share.  I succeeded in setting up the samba box as a 
member server on the AD domain and domain users can login and add files 
to the samba share but all attempts to run the backup fail with 
'Username LIME\SBS\ Backup\ User is invalid on this system' appearing in
the logs.  I believe that the problem is linked to the fact that the 
'SBS Backup User' account is disabled by default, however, the obvious 
fix - enabling the account - doesn't work because Windows kindly 
disables it again as soon as the backup process begins.
    On a temporary basis, I have given up and am now backing up the 
server to one of our Windows XP Professial workstations but I would like 
to at least understand why it doesn't work and preferably fix it.

Sorry for the rather lengthy mail and thanks in advance for any pointers.

Cheers,

James.