Yes, bad guys troll to find computers that are opened and can be hacked into. A firewall is especially important if you have cable or DSL that are always open. ZoneAlarm is an excellent free firewall that among other things allows you to prevent programs from connecting to the internet if you don't want them to, which can be another security risk.
Firewalls are funny because they can be hacked too. Hostile java will hack your browser and ip forwarding / blocking won't do a thing. Seriously, your main question should be: How many warez will i install, and how many hits per second on porn sites will you have?
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