General information about port 445
  Among the new ports used by Windows 2000 is TCP port 445 which is used for SMB over TCP. The SMB (Server Message Block) protocol is used among other things for file sharing in Windows NT/2000/XP. In Windows NT it ran on top of NetBT (NetBIOS over TCP/IP), which used the famous ports 137, 138 (UDP) and 139 (TCP). In Windows 2000/XP, Microsoft added the possibility to run SMB directly over TCP/IP, without the extra layer of NetBT. For this they use TCP port 445.

  At its simplest NetBIOS on your LAN may just be a necessary evil for legacy software. NetBIOS on your WAN or over the Internet, however, is an enormous (read foolish...) security risk. All sorts of information, such as your domain, workgroup and system names, as well as account information is obtainable via NetBIOS. It really is in your best interests to ensure that NetBIOS never leaves your network.

  If you are using a router as your Internet gateway then you will want to ensure that it does not allow inbound or outbound traffic via TCP ports 135-139.

If you're using a Firewall then you should also block the same ports - TCP ports 135-139.

  If you are using a multi-homed machine i.e. more than 1 network card, then you should disable NetBIOS on every network card, or Dial-Up Connection under the TCP/IP properties, that is not part of your local network.

How to disable NetBIOS over TCP/IP?
  In Windows 2000/XP you have the ability to disable NetBIOS over TCP/IP. You do this by right-clicking on My Network Places and selecting Properties. Then right-click on the appropriate Local Area Connection icon, and select Properties. Next, click on Internet Protocol (TCP/IP) and Properties.

Now click Advanced, and select the WINS tab.

There you can enable or disable NetBIOS over TCP/IP.

The changes take effect immediately without rebooting the system.

How to disable port 445?
You can easily disable port 445 on your computer. To do so follow these instructions:

Open regedit and go to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NetBT\Parameters

Under that key locate the entry named TransportBindName.

Double click the TransportBindName entry and then delete it's default value, thus giving it a blank value.

Reboot for change to take effect.

After rebooting open a command prompt and run netstat -an to verify that your computer is no longer listening on port 445.

Windows 2000/XP Client and Server port 139 & 445 usage
  If a client machine has NetBT (NetBIOS over TCP/IP) enabled, it will always try to connect to the server at both port 139 and 445 simultaneously. If there is a response from port 445, it sends a Reset (RST) flag to port 139, and continues it's SMB session to port 445 only. If there is no response from port 445, it will continue it's SMB session to port 139 only, if it gets a response from there. If there is no response from either of the ports, the session will fail completely.

  If the client has NetBT disabled, it will always try to connect to the server at port 445 only. If the server answers on port 445, the session will be established and continue on that port. If it doesn't answer, the session will fail completely. This is the case if the server for example runs Windows NT 4.0.

  If the server has NetBT enabled, it listens on UDP ports 137, 138, and on TCP ports 139, 445. If it has NetBT disabled, it listens on TCP port 445 only.