Who is vulnerable?
Any machine on the Internet can be reached by another machine on the Internet, unless it’s protected by software or hardware designed to control that access. Whether your connection is by dialup, ISDN, cable modem, T1, or ADSL, you can be attacked over the Internet.
Broadband connections tend to keep the same IP address for long periods, while dialup usually only has a specific IP address during a one session. This means that the villains scanning for Back Orifice, NetBus, Hack-a-Tack and others concentrate on the broadband connections. It does not mean that dialup accounts are immune to attack.
Why are you vulnerable?
Because you’re there and because of what you run. Sometimes its the software on your machine which makes you vulnerable. Sometimes its the Internet functions you like to use. Some functions in very high risk — ICQ, IRC, and “instant messaging” programs. Sure, they are cool. But, they’re are dangerous because your IP address can be visible or available and the other folks know you’re on the net. This last aspect completely negates the security advantage that dialup accounts with dynamically assigned IP addresses have over the permanent or semi-permanent addresses of cable modems. Add that to the security problems of some of these products, and you have created an easy attack route.
What’s a firewall?
It is a program that examines data packets arriving over a network and programmatically makes decisions on whether or not to allow the packet to get to its stated destination. This program may run on the machine that it is protecting, or it may be built into and run on a dedicated computer or a specialized piece of hardware, which can also provide additional isolation via connection sharing.
In other words, it may be a software that you run on your computer. It also may be your cable / DSL router. You may even use both, and I suggest you do, because both of these options have unique advantages.
Why should you be concerned?
You’ve spent a lot of time and effort setting up your computer and the files on it. Some of these may be personal; others may relate to work or investments. All this took your time and effort to create, and you don’t want to have to do it again. Just reinstalling windows will take you upwards of an hours, once you find all the drivers for your add-on hardware. This assumes the bad guys were just destructive. But, they aren’t all that way.