had the same issue when my free software on new laptop ended, I now use Microsoft security essentials (MSE) as recommended on here, I have used other free software in the past on my office pc software was called comodo internet security and it was a nightmare to be honest, now I am using MSE at home it runs in the back ground effortless, had no problems yet so I am going to use it for the office too
I look to get internet security instead of just anti virus because they often offer an all round package with firewalls built in and such, the MSE is just called an anti virus, I believe it doesn't have a firewall but is a fairly wide spread anti virus. It auto updates which is useful but it's from a name I trust which sells it for me. I use my windows firewall.
someone tell me if I'm wrong
http://www.microsoft.com/security_essentials/