Well, most suggestions I'd give have already been covered (or reversed, because I really
don't like Microsoft Security Essentials, for various reasons... however, probably not all
of them perfectly legitimate and unbiased).
Bear in mind that whatever
AV you get, something
might well eventually get past. Whether something zero-day, or specifically designed to get past one or more of the top ten AV products out there. Common sense helps. Unticking toolbar/"why don't you also try" automaticly enabled suggestions in downloads that you do
make might be good. (And if a toolbar ever does
appear, it might be signs that something else installed without you being fully aware. But then it would be even if you already had
other toolbars, so not a strong argument, that.
) And backups (themselves secure, where relevant) and knowing how to resetup your system from scratch, in an emergency would be useful.
As for level of protection, I tend to think that anything fancier than "scan executables before they get executed" always seems like more fuss than it's worth, when used within the common-sense approach. And the free (not 'free trial'!) versions of pay-for-more-functionality AV programs do that (and some
more things, even ignoring the pestering to upgrade/trial-upgrade), so I'm afraid I'd be one of the free-loaders whom the paying-for-the-whole-lot users is helping support the speedy response to new threats for.
OTOH, if you're a bit more 'out and about', I've seen some of the fuller versions have tools that you can pre-load with the fingerprints of some of your more sensitive data (e.g. credit card numbers) and keep an eye out for this information being exported (to block or 'are you sure?'), which could show an (undetected) application playing fast and loose with what it's discovered regarding your identity. Not too sure if it's a good idea to deliberately group such stuff together for the protective
program to hang on to, but hey...
One approach (not to be relied upon, and generally deprecated) is "Security Through Obscurity". Don't run IE, Firefox, Chrome, Opera, etc, but one of the lesser-known browser brands. Choose a free AV that doesn't
start with "Av" (Avast, AVG, Avira... ummm, any more?) and head down the alphabet a bit (although still get one that is good
!), or go for something like Bullguard over Norton (not so much rolled out to millions of machines, and therefore a decent chance of being what the random virus-planter's software is designed to get past). Heck, go to Linux. But, really, these are big steps, may need some re-education, and is definitely not for everybody. And, obscurity's not a solution if it isn't also a functional
solution at the same time.
Darnit, there's just so many different approaches to this problem. For the record, I'm generally happy with AVG (free version, sans
toolbars and the other free 'special features' where I can help it, although I'm checking out the Do Not Track feature, right now), with MBAM (free, non-trial) for on-demand scanning, judicious hosts-file editing to rid myself of adverts, and a whole host of tools on a memory stick that are ready to be used (in some instances, bootably!) on any
machine I need to examine closely or otherwise check. But this is as liable to change as not, and several of the machines on my desk at the moment are
differently equipped, even unto browser and various even geekier configuration settings, and (in a couple of cases) the entire family
the OS is a member of...
 But, insofar as toolbars, I just wouldn't download them anyway
. Even AVG/Norton/whatever-AV's suggested toolbar is not installed, when I have any say in it, or removed as quick as I can. And (on IE8) the "favourites" toolbar delegated to being a non-toolbar. That's just housekeeping, though.
 Or macro-able documents, browser plug-in scripts, etc, BYGTI.