Creating a credentials file will not work for Server 2003, since by default all authentication attempts need to be "secure".
Here's what you need to do:
Open up the registry editor and navigate to:
HKLM\System\CurrentControlSet\Services\Lanmanserver\Parameters
Find the RequireSecuritySignature key. It will have a value of "1". You need to change this value to "0" in order to disable the secure authentication feature and allow Samba to connect.
Reboot the server and you should be set to go. Unless, that is, you are actually running a Small Business Server 2003 in which case there are Group Policies set that will undo your changes. If this is your situation, let me know and I can help you change those also.