This page lists the best anti-virus software for Windows.
I recommend Microsoft Security Essentials. Home page at http://windows.microsoft.com/en-us/windows/security-essentials-download
For some virus, i also recommend Malwarebytes' Anti-Malware. Home page at http://www.malwarebytes.org/
But don't run multiple anti-virus software on your machine. Run just one. However, you can install several, and use them to manually scan for viruses periodically.
Here's 2 others, but i haven't used them.
I do not recommend anything from Norton, nor McAfee.
“rootkit” is the most nasty virus software. Once a rootkit infected your system, normal anti-virus software may not be able to detect it. Here's the tools to detect and remove them.
I've used the following around 2013. Not sure if the are still good.
I recommend all of the following.
For other excellent software for Windows, see: Great Software for Microsoft Windows.
〔Hunting Down and Killing Ransomware By Mark Russinovich. @ http://blogs.technet.com/b/markrussinovich/archive/2013/01/07/3543763.aspx〕