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Even with dial-up via a modem I think a firewall is a must. Mine's stopped a heap of port attacks, adware attempts and other stuff while I've been online. It's maybe a mistake to think people with modems only go online fleetingly, I'm often on for several hours researching, and in that time a lot of people can gain access to your computer. If in doubt whether you need a firewall or not, install one and run it for a while, then look at the log.