On an un-patched, un-updated Windows XP machine, maybe. From Vista onward, the built-in software firewall and UAC make a default install relatively safe. I neve

On an un-patched, un-updated Windows XP machine, maybe. From Vista onward, the built-in software firewall and UAC make a default install relatively safe. I never ran any real-time virus protection on Vista and 7 (though I did run a few manual scans just to be sure, which were all clean) on a variety of machines and never had any issues.