I can't stress this enough and I think I've said this before, but if you must use free anti-virus software for Windows there is no beating Microsoft's Security Essentials. Sure it might be unfair and not good for encouraging competition but there is no denying that it’s the fastest and most lightest of anti-virus programs that I've ever used for Windows, and its actual ability to pick up threats to your computer matches (in some cases goes beyond) many premium subscription based alternatives. It actually does the job and does it well. 

But if you look at it in the most purest logical way there'd be far less risk involved using a Microsoft's anti-virus program. Firstly because Microsoft actually have a true vested interest in keeping their operating systems fully functional and free of malware, and secondly they wouldn't be gaining any more access to your system via an anti-virus program that they couldn't already gain via the actual OS. 

That’s also exactly the problem with a lot of free (and even premium) anti-virus software, they also allow for additional access to your system via the internet. Take AVG for example, I've seen it perform in its 2012 form and it seemed to seriously bog down a 900Mhz ASUS eeePC where as Security essential's didn't slow the machine down in the slightest to keep the general idle process load below 5% overall to spike at 10% every so often even with its real time scanner active! That’s how light and fast security Essential's is. There was also the slight fact that AVG had at least 7 elements to it that needed network/internet port access in order for AVG to actually function. 

Purely from the functional standpoint of keeping your Windows machine running as smoothly and as fast as possible

Admittedly the 2013 version of AVG looks pretty slick purely from a cosmetic standpoint as its gotten a face-lift to make it look a lot like how the Windows 8 Metro is themed. But cosmetics aside will the free 2013 version of AVG be enough to trump the true and proven pure performance of Security Essentials on a Windows based machine?   

Having said all that the only drawback to Security Essentials is that you do need an activated version of Windows that can receive official security updates in order to be able to install, run and regularly fully update security Essential's on your computer. 

If there was a better alternative I would whole heartedly recommend that instead. But the fact is of all the anti-virus programs I've tested (some free and paid subscription based) security essential's outperformed much of the competition on many levels for said task.