I have great respect for Methyl when it comes to advice on Computer problems. He can certainly run circles around me.
However, if it's just a straightforward security you are seeking for your computer and would wish to have something reliable and costs nothing, then the answer is certainly MSE (Microsoft Security Essentials.)
The best free antivirus to use is AVAST for free . Microsoft Security Essentials is no longer a viable antivirus program since Microsoft has stopped doing any work on it and is asking users to use third party alternative programs of which AVAST is the best.
At the moment I am staying with MSE. However, I am seriously thinking of changing and, in this regard, I acknowledge that galeck1947 has put forward a good reason for me to change to AVAST which I understand is really good.
No way will I pay for AV when there are free programmes such as the following to choose from.