I use a combo approach.
Microsoft security essentials free
Spybot Search and Destroy. Free but worth donating for the cause.Use the "Immunize "feature and keep it updated.
And last Super Anti Spyware. Free
I own my own computer co. and have sold computers and computer parts for 15 years and my husband is the IT Manager for a large eye institute. I beta test for MS and can tell you that you NEVER use more than one anti-virus on your computer. Just saying.... Avast is a 5 star program and we use it on our 7 computers at home and I install it on our customers' computers. You have the best with Avast and if you read about the two, you will find they do the same. I DO NOT like MS essentials. Ran it for about 2 months and it does not catch everything.
Warp 2 Computers