Ad block plus isn't an anti virus..
Open source OS's ? What does that have to do with anything. ANY OS can get a virus... Windows are just more likely as MOST people use windows, and there's a larger 'market' per se, for malware and shit.

Anti Viruses don't necessarily lower performance if you get a decent one... Perhaps during the actual scan, but it won't kill you to take two hours out of your day to scan (or just leave your computer on over night).

I HIGHLY recommend BitDefender (Total Security)

MS Essentials isn't great, it's light, useful for netbooks, or low-performance computers.

MalwareBytes' AntiMalware is a must have, even if you already have another AV, it's got full AV features in it's FREE version, and a couple additional things in the Pro version which aren't necessarily needed, but quite useful.

Trend micro has great internet security.
Kaspersky and eset are also two highly recommended AVs, but again, my personal recommendation (after using many AVs) is BitDefender.

Then there's other things to deal with :/ I keep a copy of Super Anti Spyware (Portable) on my PC, run it every so often (it's nothing you leave running or anything, so it doesn't lower your performance or w/e). Great tool.