When working with old SCO Unix servers that run Samba I have found an issue since a Microsoft security update KB2536276 was released to fix a vulnerability in Windows.
MS11-043: Vulnerability in SMB Client could allow remote code execution: June 14, 2011
This update causes a problem with older versions of Samba that make shares unavailable to users.
The only way to fix this is to remove the KB2536276 update on the Windows PC.
First remove the update from the pc in Add/Remove Programs. Find KB2536276 and remove it. Then in Windows update ensure that you stop the KB2536276 from being reinstalled.