I agree with using Microsoft Security Essentials. I run a computer shop out of my home and put it into every computer. Have not had any issues and does not nag or try to trick you into switching to paid version. I find it works very well.
I used MS Essentials and it works great. I used to use AVG and MS Defender, but MS Essentials is much easier and with Microsoft Update set for automatic updates, it works flawlessly. I have switched all 3 of my PC over the MS Essentials.
I've been running it parallel with Avast for a couple of months now, and seems to be working pretty well. When my Avast expires later this month, I'll run MS by itself.
As a bit of added advice, It is always recommended that you uninstall your old Security software (AVG) before install the new product...sometimes the old Virus Signature files used by the old anti-virus tricks the new sofrware into seeing those files and assumes you have a virus.
MS Security Essentials works fine for me. It does tell you to remove other virus programs before installing. I use Avast pro on one and MS on my laptop. Never a problem with either.