[rcheu]

I don't have Adblock on for Youtube and I've never gotten a popup like that. All of their ads are video ads. Are you sure you don't actually have a virus (that's causing the popups, not due to the message itself)?

[kuschku]

The in-video popups (at the bottom, about 20% of the height of the video) very often advertise malware for me, too.