I don't use any antivirus because I use Linux, and Linux is natively secure, and doesn't really need one.
But before I chose to use Linux, I was on Win7, and I used AVG Free in conjunction with spybot search and destroy. However, I cannot vouch for them as they let in two viruses before. The first one prevented AVG and spybot from running, and requested that I buy their (probably fake) antivirus. Had to do a clean install to get rid of it. The second was a boot sector virus. I no longer had my copy of win7, so I installed Ubuntu as a dual boot for a while, took some files I couldn't replace from my windows partition, then deleted windows. And I wasn't doing anything "shady" on there. If you run windows, you need several layers of antivirus protection just to keep it running, which is the main reason I will never go back to windows.