Microsoft essentials does the trick if you keep it upto date. AVG and that sort tend to slow your pc down with crap
Avast is the best, least trouble and free. http://www.avast.com/en-gb/index AVG can give problems, Microsoft can too. Trust me, Avast.
I think a lot of virus protectors/ firewall vendors do a 'free' version but its best to check as what you are protected against as I find they all try to flog you the 'Pro' version.
Avast let a virus onto my computer causing a loss of all my work stuff ( thankfully I had backed up to disk ), like I said Mse has never let me down and regularly updates itself and comes with no hidden additions that I can find anyhow ( 30 years computer experience ). However each to their own I say.
I use MSE, its generally pretty good and had no problems with it. I've always thought the best way is to just becareful what you go on and don't do daft things. My mam uses my laptop and has a habit of just clicking on anything that pops up
The Microsoft Essentials software seems very effective and easy to maintain - is a long standing Which "Best Buy" (even though it's free!). I've used AVG in the past and that was also pretty good.
I've used Avast for years, and never had anything get through, it always stops any threats for me. MSE Im sure is fine, I wouldnt touch AVG or Zone Alarm with a barge pole now. (15 years IT Security). In all honesty if you rely 100% on ANY AV to keep you safe, you're mental. You still need to apply common sense, dont click on stuff in emails you aint expecting, avoid dodgy sites etc etc.
I have had trouble recently that I asked for help with. I thought I had a virus that was cross infecting my pcs. I use five. Don't ask why it just happens ok. But the problem was actually not a virus. It was simply my modem had an issue and this triggered error messages that are normally associated with viruses. Always check out your modem before fearing the worst, forget yahoo mail, and change your passwords every month.