I have recently started using Emsisoft. I read that Microsoft security essentials doesn't do the job anymore and Avast is getting resource hungry. Emsisoft has a free version and a paid version (I think I paid about $30 for use on up to 3 computers) and includes anti malware as well as anti virus and is very highly rated. I've been very happy with it.
http://www.emsisoft.com/en/