I usually never install anti-virus, because I tend to use add-ons for Firefox like NoScript and AdBlock Plus. They aren't perfect, but they can usually avoid malicious ads, which
can appear on sites like Anandtech. Actually, didn't Anandtech have a malicious ad on the site a year or two ago? Sometimes they slip into the rotation.
I did get an odd message sent from my Twitter account to my friend, but I know I did not send it. Curious if maybe my methods of prevention and tech know-how had failed to block an intrusion, I installed the free Microsoft Security Essentials, and all it found was the mIRC installer (it has some tool built-in to allow it to present a piece of software to install in tandem with mIRC -- optional of course). So, I had no actual problems. I don't mind having it installed though... I just turn off automated scanning and let it sit there.
I also don't use a software firewall... usually the fact that a router tends to block unwarranted traffic works well enough for me.
EDIT:
it will miss the schedule time every single time because nobody leaves the computer running at 3AM!!
Three of my computers are usually running 24/7; however, I turned off automatic install and told it to only alert me to updates being available.
Although, I installed a few updates yesterday, and it was odd... my machine sat there on the "Shutting down..." screen for about 10 minutes . I literally walked over, updated my server, restarted it and had it back on the desktop and my main desktop still wasn't done. I just powered it off incorrectly, but I'm wondering why. I used to have an old machine that did this (it literally took 5 minutes to restart), and the problem went away after I replaced a video card (I think).