Sharing Error 1219

My boss's machine won't connect to our "server". I'm trying to connect his Windows XP machine to a specific share on a Windows 2000 Professional machine, and when I try, it says "System error 1219 has occurred.", etc, etc. I realize this is an issue with the credentials.

1) If I disconnect all of the other shares on his machine, it will work, but then I can't reconnect the other shares.

2) I tried a "net use m: /delete/yes", and it says it's not connected, but still won't connect.

Any further ideas?