This page lists the best anti-virus software for Windows.
Here's a list of good anti-virus software.
I recommend the Microsoft Security Essentials. Use that.
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 Norton AntiVirus or 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.
The software from Piriform are excellent.
For other excellent software for Windows, see: List of Great Software for Microsoft Windows.
See: 〔Hunting Down and Killing Ransomware By Mark Russinovich. @ blogs.technet.com…〕