One of the better "FREE" ones is
Microsoft Security Essentials. It also has malware built into it.
BTW: Only run one anti-virus program at a time, so if your not going to use AVG, then uninstall it. I use Security Essentials on all my systems.
After you install Security Essentials run a scan. To remove AVG:
Start | Control Panel | Programs and Features find AVG click on it then at the top click uninstall.