SMB Error -41

We have a Windows SBS 2003 server. When a mac 10.4.7 copies files over to it they sometimes get an ERROR -41. It happens on random files of different sizes. It seems to be when the file size is large it happens more often. We copied the same folder to an XP pro machine and the issue didn't occure. I have tried changing the lanmanserver registry setting on the SBS 2003 server. All SMB signing is disabled etc? Any ideas?