PORT 139

139 is NetBIOS....and to quote:

"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"

Take a look here for common port info:

http://www.iss.net/security_center/advice/Exploits/Ports/

For Internet related Ports:

http://www.chebucto.ns.ca/~rakerman/port-table.html

Delete any spaces in the URLs. Hope this helps!

To add to Alpha-Male's very correct answer, I suggest you go to GRC.COM
Click the SHieldsUp link.
On the next page, click the ShieldsUp link again.
Click the Test My Shields link.
Read and follow EVERYTHING you see from then on.

You MUST close TCP port 139 on a Windows box. Must.

And I'll add that after you go to that site and scare yourself silly, download the firewall he recommends. It is easy to use and blocks all your ports beautifully without you having to go through all the hassle of actually shutting down the netBIOS port, which can be a bit of a pain.
I show up as invisible now. :)

Nikki

AHA GRC.com crowd! Yep been there reading the scary stuff too.

To add a couple of cents, if you are not connected to a local network using TCP/IP and if so as long as you're not sharing files or printers, NetBios is not in use, therefore port 139 is not open.

It's better to share using NetBEUI to explain in simple terms this protocol does not show up over the internet.

Here's the site at GRC.com where you can test how secure you're system is.

https://grc.com/x/ne.dll?bh0bkyd2

This question was closed by the author