Port 139 NetBIOS

NetBIOS Session (TCP), Windows File and Printer Sharing

This is the single most dangerous port on the Internet. All "File and Printer Sharing" on a Windows machine runs over this port. About 10% of all users on the Internet leave their hard disks exposed on this port. This is the first port hackers want to connect to, and the port that firewalls block.

Microsoft Ports