In the early 2000’s, my first computer was infected through a “codec pack” that came in a CD that a colleague gave me. I remember to see some programs not being able to start, first Windows Media Player, then others. I was using Norton AV (that came pre-installed in the computer), that was able to detect the virus after a on-demand scan but not in real time. I also remember that some virus added itself to the exclusion list of Norton AV! At the time, Spyware Doctor from PC Tools, Kaspersky AV and the young Webroot SpySweeper, were highly regarded anti-malware applications.
All of mentioned, as far as I can tell. When I got the internet, my PC used to be like a Petri dish for malware, but that is a history, I guess.