Having been broke the last few years I had to scratch around for free stuff. Thought I'd share my experience for what it's worth.
Been running for at least two years with a combo of Webroot Secureanywhere (not free but I keep running into keys on the inet) and Avast free.
The two have kept me clean and have had no impact on performance. Very light and great detection.
Webroot is a tiny cloud based antivirus with virtually zero impact and excellent detection rates. Very impressive product. Works differently in how it detects threats to other antivirus programs. I tried Bitdefender and ditched it quickly. Same with Kaspersky.
Here's a free 6 month trial for Webroot.
Because it is a cloud based av it plays well with other av's. No problems together with Avast.