Sadly, there are a lot of dinks out there with too much time on their hands and anti-social personality disorders. More than a few of them spend a lot of time “port-scanning” blocks of IPs, looking for vulnerabilities, active trojans, what have you. If you’re on dial-up you’re something of a moving target, you connect for an hour (or whatever) then drop off. The next time you connect, chances are you have a different IP. Also, you’ve got way less bandwidth, making you not as valuable for certain purposes. DSL and cable are “always-on”, have the same IP for large blocks of time, and have a fair amount of available bandwidth.
Firewalls do three things. The first is that (presuming for all three that you’re using a decent one and it’s properly configured) they make you essentially “invisible” on the net. To a port scanner it looks like the IP isn’t in use, makes it look like there’s no computer there. The second thing is that they can protect you from some kinds of attacks. The third thing is that they only allow authorized programs to access the net - useful in stopping trojans, malware, and various such annoyances from being able to phone home.
There are two kinds of firewalls. The first is hardware based, these days most routers have a built-in firewall/NAT server. A properly configured hardware firewall makes things extremely difficult - darn near impossible - for an outside attack to effect your internal network. The only problem is that a hardware firewall can’t do #3 above. Since the hardware firewall has no way of knowing where the packets are coming from on the internal network, it can’t limit what programs can access the net. The other type is a software firewall that runs on your system. A software firewall isn’t quite as good at protecting your system as a hardware firewall, but since it is running on your system it does know what programs are attempting to access the net and it can (and should) limit that to those programs you have identified as having reason to do so.
As far as software firewalls are concerned, there are a number of them out there, many of which have a “free” version. ZoneAlarm is probably the most popular because it’s extremely easy to use. Myself, I don’t like it much, it’s a resource hog and I’ve had problems with it in the past. Be that as it may, if you’re not familiar with how things work on the 'net, ZA may be your best choice.
Anyway, a few of the popular software firewalls with freeware versions -
ZoneAlarm
Sygate Personal Firewall
Agnitum Outpost
Once you’ve got things set up Gibson Research has a test called “Sheilds Up” that will scan your system for vulnerabilities. There are several such tests out there, it’s worth trying them before and after to see what it is your firewall is doing for you.
A couple of notes - WinXP has a built-in firewall, but it’s not worth much as it doesn’t give you any way of controlling outbound traffic. Also, the tests above are not worth a lot if you’re sitting behind a hardware firewall.
FWIW, hope that helps a little.
Myria