I don’ t recommend a specific brand of anti-virus since no software is fool-proof.
For Windows computer I recommend having security /antivirus software but don’t recommend a specific brand.
For Macintosh and chrome, and currently smart phones I have no recommendation, since sometimes it can actually slow down performance more than a security concern is worth. For those, I say if you want it on there, and it works well, then that’s ok.
Another thing is to remember avoid browser plug ins, toolbars and extensions unless you know what they are for. Avoid downloading programs that you don’t know if you can trust and be careful installing so you don’t add more than you want.
Be very careful about browsing. Avoid free video and music download sites unless you know they are legitimate. Avoid bit torrent and peer sharing apps. Avoid pornographic sites. Also legitimate sites that are not well maintained can be a problem as well in the comment and discussion boards.
Another thing to remember is most computer problems aren’t viruses. So you may want to use builtin programs like disk cleanup, and also go to control panel to uninstall unwanted programs.
Other problems aren’t software and are actually hardware. Your computer may need to physically be cleaned out of dust or better maintained physically. It may need hardware upgrade such as more memory or a solid state drive, or if it is real old a replacement which may be another computer or a tablet.