It might not be Samba.
Windows can sometimes send the wrong information when an account name as been changed.
What could be happening is that you think that "bobf" with password "aM2133" is being sent, but the user name being sent by Windows XP is still "Administrator" or whatever the previous name was. If you don't have an "Administrator" account on the Unix side, or if it has a different password, you won't get access.
I have a longer explanation at http://aplawrence.com/Unixart/winauth.html
but the short version is to take a look at the account with
If not that, the Samba logs can give you a clue. If you see an error you don't understand, post it: odds are that somebody here has seen it.