2003 Small Business Server backup.

Hi,
Hoping you can help.
Im having an issue with small business server backup using the /UM switch.
When I add the switch under scheduled tasks it asks for the NT AUTHORITY/SYSTEM password which I dont have. So I change it to use the Administrator account but after a little while it changes back to its orginal. ie. NT AUTHORITY\SYSTEM and without the /UM switch.
Can you please tell me what I have to do?
Thanks in advance.
Silvio.