I have always had a slightly ambivalent relationship with AV tools.
My main desktop machine is more used by my wife (who routinely hits the "temporarily allow everything on this webpage" option in NoScript, sadly) so I have a paid AV package on there (eSet NOD32).
This year, they offered me an almost free update to the full Internet Security offering. I resisted: I have yet to see one of those that doesn't cause more problems than it solves.
My netbook has had MSE for the last two years but, amidst odd reports of it being less secure than it was (because it's become a big enough target, I guess) I've just replaced it with NOD32 too (the second license was reasonably cheap).
Interestingly, dcupdate works a heck of a lot faster now MSE's gone.
So I still use the Malicious Software Removal Tool, even though it's never found anything and makes Patch Tuesday more of a chore than I'd like
and Defender's on both systems -- NOD32 replicates quite a lot of its functionality, I think, but they seem to coexist happily.
MSE did find something, once, on my netbook: an infected JPG on some webpage or other, if memory serves. NOD32 seems a decent-enough product, the support is at least reasonably responsive, the program's very configurable, not very obtrusive and seems effective.
I hadn't even heard of the offline version of Defender before reading Fred Langa's article yesterday. Nice to know it exists, I guess. Ditto the Safety Scanner.