For a free basic antivirus solution, we recommend Microsoft Security Essentials.
If you purchase items online or use online banking, we recommend Norton Antivirus. In the past Norton was notorious for slowing systems down, but nowadays it is a lot faster and will not slow down your computer any more than McAfee or most other antivirus solutions.
To protect your system from the latest threats we also recommend Malwarebytes. You can use their free version and it will protect you from the latest malware threats. The advantage of the paid version is that the program will update automatically and will actively protect your system against threats. Using Norton and Malwarebytes together will protect your system better than any other products on the market.
One thing that should be avoided is installing two anti virus solutions. This is a common mistake, so before installing a new antivirus, make sure and uninstall the existing antivirus solution.