MS security essentials is what I use . I do virus research as a hobby , I keep a pc that has no protection, just an imaged backup ,that I infect with malware/rootkits/virus on purpose to see what they are doing, then I disassemble them to see how they work.
Most virus used to be vbscripts but that has changed especially in the past year. An idiot programmer decided it would be cool to create a virus creation tool that requires no programmer knowledge. It is a menu and check box driven program that allows someone to run it, pick what they want the virus to do, then it creates the virus program and can even scramble and obscure the code so AV programs cannot easily find it. That program has caused a flood of new virus to appear since now non-programmers can create them.
I like MSE since it doesn't get in the way. It isn't as light on resources though as many think. If you are just looking at the Mssecs.exe that isn't the amount of resources it is using. That is the interface and menus for it. Instead look for MsMpEng.exe , that is the actual program, for me that is 148MB . Still not too much for me not to use it.