Greg keeps getting viruses on his computer, and wonders if there's a PC cleaner that would take care of this. Leo says no, there's no cleaner that will successfully remove all malware. Greg says he has a computer guy that cleans the viruses off, but they keep coming back. That raises concerns that the technician isn't getting them all. Once the wall has been breached, viruses can be inviting others in. Greg says that his technician has been reformatting and reinstalling the operating system, which is good.
Leo says that every security expert in the world will say the only way to be sure the viruses are gone is to erase the hard drive and reinstall Windows from a known, good source. Then run all Windows updates.
It all comes down to user behavior. He can have antivirus and cleaners such as MalwareBytes, but unless he avoids sites and services where malware lives, he's going to keep getting viruses.