There are obviously other solutions out there but for all those that own a legit copy of Windows I'd say Security Essentials is your best 100% free solution which I can't stress enough if you are indeed using windows on your machine.

I'm not sure what the big deal is but as far as 100% free anti-virus protection goes MS security essentials is pretty efficient with a real time scanner for all incoming files whether compressed or uncompressed once installed. It'll even pick up none too serious threats after which you then have the option to just ignore if you know better. The fact that its official doesn't make it anymore untrustworthy as what would Microsoft have to gain from an OS that they already created? If there was anything malicious about it surely they would have just included it in the OS to begin with?

Then there's the fact that security Essential's doesn't seem to noticeably effect system performance in any way when compared to how it ran before it was installed. I.e. Your machine is still as fast as it was before you installed it. It will also automatically download and install the latest patterns regularly for free.

The important part is that security essentials uses a real time scanning engine. Meaning it will automatically scan things as you use them and the fact that it does it without really affecting performance is the best part. Other free alternatives don't have a real time scanner meaning you have to manually scan your entire drive or particular files as you use and download them. If you don't perform the manual scan with the free virus scanner without the real time scanner, it won't tell you a thing as the virus spreads through your system.

Here's the thing as they mentioned on recent episode of digg I watched. If people aren't opting to install premium anti-virus software it basically leaves the whole internet more vulnerable to the spread and perpetuation of viruses in general. Security essentials doesn't nag you with loads of crazy pop ups that you just don't want to see like alot of the premium AV software out there nor does it run out provided you have a legit copy of Windows installed. It will only tell you if it has automatically detected a virus without you having to run any manual scans to actually find them. I just don't get why Microsoft are under fire for it since people obviously don't want to pay for premium virus protection which as I mentioned before makes the whole internet a far less safer place.

Not that I'm endorsing it but all those that can't validate the copy of Windows, they  obviously won't be able to use Security essentials since it basically won't let you install it and if it is installed it will then disable it. Which is where the free version of AVG comes in very handy. However although really light and resource sipping it doesn't in fact have a real time scanner unless you upgrade to the premium/pro version of AVG.

From personal experience some of the main providers of premium anti-virus protection do actually make your system feel noticeably slower after you install it.