Is there really a reason to even run anti-virus software all the time as long as you don't try to open executables and macro-containing documents that didn't come from a reputable source?
Among other things, I review orders for an advertising service, 20-30 a day. Some of these orders are purposely placed to advertise sites with malicious code that installs malware. My fully patched Windows 7 system behind a firewall, running antivirus and the latest Google Chrome, gets infected with something or other on a regular basis -- at least once a month -- without me ever downloading any files.
Last week it was one of those fake antivirus programs that terminates all your real antivirus programs and pops up a window saying you're infected and need to upgrade for $29.99 every 20 seconds.
That one was probably a Java plugin vulnerability.
That's not entirely accurate. Most vulnerabilities do get patched quickly.
It's still not worth enabling it in your browser though, because so few sites use it.