On my Pro 1, which I have had since the day it was released (Feb 2013) I depend on the built in Microsoft Security Essentials and the Defender. This is my main computer which I use for several hours every day all over the internet. I have never had a virus, (knock wood to appease the spirits of the forest) so far. Malware, which changes my home page or my search engine, is easily removed when it rears its ugly head. I have two more laptops that I use on occasion, and my several friend's computers. None of these devices have ever had a serious malware infestation if they were running Microsoft Security Essentials or Windows 8, which has it built in to the os.
Microsoft Essentials is basic, and not enough for good protection. I use Bitdefender 2014 on my home PC and on my SP1 pro. It is fast and lot of protection options.
For about $15,- you can buy a 1 year licence.