Not sure if already added but thought of sharing as is really good and 100% FREE antivirus and anti-malware from Microsoft.
This has been there for quite some time and i have been using this for long and is really good and reliable.
Use Microsoft Security Essentials to guard against viruses, spyware, and other evil software. It provides real-time protection for your home or small business PCs.
Microsoft Security Essentials is free* and we designed it to be simple to install and easy to use. It runs quietly and efficiently in the background so you don't have to worry about interruptions or making updates. http://windows.microsoft.com/en-GB/windows/products/security-essentials