I don't use an anti-virus and I disable Windows Defender. They slow down the system too much. I use my linux laptop to if I want to do anything questionable. I also tend to reinstall Windows about every 6 months.
I use Kaspersky 2013, one of the better AV's out there. Safe practices and a free AV like MSE will prevent a lot of headaches anyways.
Yes, on old single core computers it's virtually unusable due to it's resource (CPU/RAM) requirements. On a modern multi-core computer with sufficient RAM- you won't even notice Norton running. In the end- as has been mentioned in probably every thread ever posted regarding "the best" anti-virus/malware software- they all let some nasty stuff in occasionally. The idea is to keep as much out as you can- there's just no way to keep it all out unless you're a savvy user. That being said- when customers bring me infected machines and I'm done cleaning them... if they are willing to pay for it; I recommend Norton Internet Security. If they want a free solution (or have a older single core CPU) they leave with Microsoft Security Essentials. To put my money where my mouth is- I pay for and use Norton at home on all our computers and also use Norton on all the computers at work. IMO- Microsoft Security Essentials is the best free protection you can get. I'm grateful that Microsoft has stepped up their game and has made Security Essentials a 'real' anti-virus/malware software now. I agree with others that if you're careful, Microsoft Security Essentials is all you'll need for a non power-user. FWIW- keygens have been mentioned- just run them in sandboxie and you'll be protected.
As many have stated already windows defender in win8 is the same as MSE in win7 MSE has never let me down so I will stick with Win Defender used in conjunction with Malwarebites pro on my win8 installation
This is probably the best option, but most can't do this. I used to reformat every 6 months, but I keep running out of HD space (6.5tb and counting) and it's a pain to move things around. However I really need to do this within one month as my PC has been too slow for my tastes. Of course if money wasn't an issue I'd just build a new PC (with GPT of course) so I could get two 4tb hard drives along with at least one SSD for booting (maybe a second just for cache). I've used the free Sandboxie program for years and love it. I plan on purchasing the paid version once it makes it to it's next major version release. I manually install Firefox within it, actually have over 5 that I can run at the same time. I use all generally for different topics, makes life easier, well once you create a note of what you use them for and finally start remembering. It's easy to move things in and out of the sandbox and it's great for testing. As far as AV is concerned I'm using Avast 7 free now on Win7, but for Win8 I might just use?? I don't always use Sandboxie, I've been meaning to install Thunderbird in there also, that would give me a fair amount of piece of mind. However, I'm going to purchase another 2tb hd and get everything organized and then go buy Win8 and then I'll reformat every 6 months again. I will use Malwarebytes and SuperAntiSpyware also, I'll try to run one of them every 2 weeks and alternate. If I get a paid AV, it's going to be Bitdefender or Kaspersky. Will probably stick with a free AV though. Going to do more reading in this forum and also on WildersSec.
F-Secure IS 2013. the best AV in the world(IMHO) see at AV Comparatives site (Real World Protection Test) October 2012