One of the best antivirus software is AVG. You can download the free version of AVG and it is a basic version that will protect your computer from virus and other threats.
Try to install Microsoft Security Essentials too. It is completely a free version from Microsoft. It will protect against Virus, Spyware, Malware and other unwanted programs.
Apart from AVG and Security Essentials you can many other free antivirus software, but AVG and Microsoft Security Essentials are good to protect your computer from threats.
Other than Antivirus software you can enable the Windows Defender and Windows Firewall will give additional protection to your computer.
Download AVG from the below path
Download Microsoft Security Essentials from the below path
Remember that Microsoft Security essentials will check the Windows copy and if your Windows copy is original then it will not allow you to install Security Essentials.