Following is just quotation of two different sources which I used to successfully disable port 445 on Windows XP machines. I was closing port 445 and 135, 137 - 139, so I followed all instruction in the article and it worked for me.
General information about port 445 (Edit: The link does not point to this article anymore).
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 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 port 445
To disable Port 445:
Add the following registry key:
Name: SMBDeviceEnabled Type: DWORD (REG_DWORD) Data: 0
Don’t forget to restart your computer after disabling the above ports
for effect. Also, to check that those ports are disabled, you can open
a command prompt and type netstat -an to confirm that your computer is
no longer listening to those ports.