Guys who know, would the big bad problem they were talking about be blocked by a firewall?
Depends on the firewall and how it's configured.
Yes, assuming those ports are blocked from the outside world.
Some ISP's (including Comcast, I think) block traffic on the Netbios ports such as 139 already, before it even reaches you.
At a minimum, anyone with a broadband connection should have a private IP address. This would stop this type of thing from even getting to your computer.
If using Windows 95/98/ME, click on Start/Run and type "winipcfg" and see what IP address is assigned to your network card. If using Windows XP, click Start/All Programs/Accessories/Communications/Network Connections and right-click on Local Area Connection and choose Status, then go to the Support tab.
I forget how to check for NT/2000 via the GUI, but opening a command prompt and typing "ipconfig" will show it to you on NT/2000/XP.
If your IP address starts with anything other than 192.168, 10, or 172.16 through 172.31, then you have a public IP address and could be vulnerable, not just to this thing, but potentially other things. If you do have a private address, it is less likely you are vulnerable.