The best free antivirus software is better than most of the commercial software available. Some commercial antivirus software have big system performance hits that slow down your computer. They do background scans, email scan, web scans, and other scans that are mostly unnecessary and hurt computer performance. However some of the free software is better optimized to allow your computer to perform better. The best free antivirus protection is still AVG antivirus software. There are some other very good programs, but AVG basic free is optimized for fast performance and with automatic daily updates it is one of the most highly rated software programs ever. There is also a 30 trial Pro version, but the basic free is all you need for good protection. I only use antivirus software to alert me of a problem. Once compromised or infected I restore my operating system using an imaging program. More important data is kept on a second partition or stored externally. So when the need arises I can quickly restore the operating system and be back to a known good configuration within a few minutes.
There are several good products, some are free, available for restoring a computer. A good free Open Source product is called Partimage which is a clone of Norton Ghost. Partimage is a Linux utility and can be found on most Linux Live CDs. I have created several Linux Live CDs with some of my favorite system restore utilities. I also like Norton Ghost DOS version which can be added to a DOS bootable CD. DOS is still my favorite utility because it is so fast and easy to use. I use it to keep my lab running 24/7/365 with 99.9% availability in a high school classroom where the students have admin rights for doing their labs on all the computers. Some students do abuse the privilege, but it only takes me less than 15 minutes to restore all 35 computers to a fresh configuration.
When my students ask me why I bother with DOS, I tell them, ” Windows is the problem but DOS is the solution”. I still teach troubling shooting but I also remind the students the time you take to trouble shoot software that is millions of lines of code and full of problems could be better spent researching and studying for your next certification.