I'm using Microsoft Security Essentials and it seems to be working fine with no problems. I used to use Avast and Spywareblaster but read that Microsoft Security Essentials is lighter on the system than having two separate types of protection. Also use Comodo firewall.
I've tried AVG, Avira, Avast and MSE at this stage and there is nothing wrong with any of them. Also had Comodo for a while but got rid of it as I was driven demented by its pop-ups.