The Installed M.D.A. Not Detecting Infected Genuine Websites?

While entering only genuine websites as advised by security experts,the websites had hidden computer viruses that were not stopped/detected by the M.D.A., from going in, and they all got downloaded plus installed without pressing any buttons,once the installed computer viruses went in it,it messed up the computer within seconds,making it unstable and very difficult to use.A full scan with the M.D.A. had not found anything as shown on the screen.I did though notice after using another free antivirus that it was showing major alerts on the screen before entering genuine websites,all detected computer viruses were removed,as stated on the screen, and the computer was spared of any kind of damages.The rival installed free antivirus has been showing to always block entries into genuine websites ,making it impossible to download a newer version of a program?.There have been cases where a full reset had to be done as the only way out of the unusable/unstable computer after a computer virus went in and never found a way to get it removed successfully,so a reset can be avoided?,any advice?