With all of the software you download, how do you protect your computer?
How do you do it without excessive system overhead?

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I use a combination of a firewall (2-way with lots of prompts), a real-time antivirus (with searching .part and .!ut files turned off), and an intrusion detection system (again lots of prompts).

Right now, I am using software all from PCtools.

The antivirus has some oddities, like it assumes that something is wrong if archives are nested too deeply. Of course, every keygen or system patch (like fixing the half-connection limit) is a threat.