What spyware blocker should I get for the iMac G5?

In general, viruses, trojan horses, and spyware are less of an issue on the Mac than they are with Windows systems. Most Mac users never encounter such problems.

However, even if you think that you don't have sensitive information, it is still very important to make sure that you follow common sense and basic security precautions. Ensure that the firewall pre-installed with MacOS X is active and configured properly and that MacOS X is updated regularly using the "Software Update" feature under the Apple menu.

The built-in firewall can be located by selecting "System Preferences" under the Apple menu, clicking on "Sharing" under "Internet & Network" and clicking the "Firewall" button. Make sure that none of the check boxes in the "allow" list are selected, click "Start", and double check that the firewall is subsequently listed as "On".

One of the easiest ways to detect theoretical spyware, as well as trojan horses, would be to consider installing a "reverse firewall" such as the popular Little Snitch which "prevents applications from 'phoning home', protects you from trojans, worms, and other network parasites, and shows which applications send information over the Internet".

The Little Snitch website notes that the product is different from a conventional firewall:

Contrary to incoming connections (which usually go to a fixed port), outgoing connections come from random port numbers. This makes it very hard to filter them in an ordinary firewall. This is where Little Snitch fills the gap: It allows you to filter connections based on the application which attempts the connection. And to make it even more handy, Little Snitch can build the rule set interactively: It pops up a dialog when an application tries to connect and asks you what to do.

Since Little Snitch and conventional firewalls fulfill complementary tasks, we recommend that you run both: Little Snitch for connections originating at your own computer and the conventional firewall for attacks from outside.

Little Snitch is available for US$24.95 direct from the developer.

An article on O'Reiley's MacDevCenter entitled "A Security Primer for MacOS X" covers keeping MacOS X up-to-date, configuring the built-in software firewall, and using a reverse firewall, as mentioned above. It also provides advice on staying up-to-date on security information, ensuring the local security of your system, basic common sense tips on how to choose a good password, using a hardware firewall, and antivirus protection. This in-depth article makes a great "first read" to begin exploring how to keep your Mac safe.

Even with these modest tips, there is no guarantee of security for any computer system, particularly one connected to the Internet, but following common sense and basic security procedures are helpful in keeping your system safe from spyware.