Does your samba user and windows user have the same password used for both? As Windows will pass these credentials to the samba server and if they do not match you will encounter this problem.
Just a little nag thats been bothering me and trying to get to the bottom of it, when our users logon to there Windows PC's they can access our Samba Share server via A Shortcut on the desktop, but IF the samba Login Username is the same as the Windows User Name it won't let that user login, why is this ??? Is it because it's trying to bind the two usernames ??