Port 137, 138, and 139 are used by Windows for NetBIOS network communication. Netbios is a somewhat strange protocol, in that it is not a true protocol in and of itself, and it isn't part of the TCP/IP family of protocols, though NetBIOS _does_ work closely with TCP/IP in enabling applications to communicate over a network.
It is a common port scanned by script kiddies. As long as you have all of the current updates you should be fine.
Short for Network Basic Input Output System, an application programming interface (API) that augments the DOS BIOS by adding special functions for local-area networks (LANs). Almost all LANs for PCs are based on the NetBIOS. Some LAN manufacturers have even extended it, adding additional network capabilities.