Just FYI, you can still get attacked by worms even if the only thing you do is plug your computer to the internet. Having automatic updates prevents this, but for my security class we tried putting a computer on XP freshly installed with no updates. You'd be surprised how many bots there are just scanning for old shitty computers.
That being said, shocka you already have a lot of protection tools that you're mentioning. Granted, they aren't as resource intensive and extensive as virus scanners and what not, but they're very powerful tools. Without those options (ie you use IE6 or something) you can easily get problems.
That being said, I only run MSE on my laptop and computer right now. On my main computer I run Comodo, no anti-virus (because fuck, some versions of avast were fucking annoying). Most viruses nowadays are Trojans anyways since they want to make a profit and the ransomware generally doesn't get on your computer without downloading suspicious files (through browser exploits or bad torrents). Comodo's mostly useful for just closing unnecessary services that windows likes to run, but also occasionally protects against worms.
So yeah, anti-virus unnecessary with a decent firewall and browser protection plugins like noscript/adblock. Still, I'd run MSE just in case anyways, though maybe not out of the way to catch the advance stuff.