I am going to give you my WACKO scenario, you may send me to the sanitarium if you want:
I run a hardware firewall (sonic wall) to protect my LAN.
I have turned off my Windows firewalls (I consider my LAN safe, seems like hassles to me).
I don't run any virus can (another hassle to me).
I just found out Win7 automatically runs defrag and am furious it does it by default and have turned it off.
My thinking is, for the past 15 years, I cannot be paranoid about getting malware, some fly will get in, as paramount leader Dend Xia Ping used to say, as long as I have a cure (my saying
My cure is to maintain few generations of OS image backups. This serves multiple purposes, not just to protect against malware, but also to protect against myself, because once in a while I will install something, just to try it out, and despite that they all say I can uninstall them, I never trust that they are all able to do so, potentially leaving garbage on my drive/registry.
When I installed my OS from scratch, (it takes like 30 minutes and it needs to be attended), I made an image backup right after that, that's my generation 1. After that I have made about 5 generations of OS images, to the last one which is the last stable and working with all the App I need. Updates go awry, bad drivers happens, this method saves you all, GUARANTEED.
Now anything happens, feel weird, i just restore my last gen OS image and am back in business within 15 minutes. I don't spend no time trying to figure out what's wrong, getting too old for that. Is like a time machine, is like doing a reset on Commander Data on that episode of STTNG to rid his brain of alien virus.
Defrag, well depends that you are doing, but I myself would like my drives to last as long as possible, and not accessing them unnecessarily. Last time I did a defrag manually, it took forever, drives are getting bigger so it figures. So my SSD boot drive doesn't need defrag, that leaves my audio/video collection drive, materials of which rarely get deleted, only added, so no defrag there necessarily either, then my third HD is my "Tivo" drive, programs which get recorded, viewed, and soon deleted afterwards, I don't feel any lag or freeze or nothing, the drives are capable of 1.5 gbit/read rate and I actually measure my single streaming rate never exceed 10 mbit, so am well below saturation, am not gonna worry about the r/w head doing some extra seeking once or twice before I delete that old recording.
YMMV.