QUESTION: occasional browser window opens to yotube.com

Hi all

some days ago a friend of mine wanted to show me a website on her
laptop and when she was about to do that, an IE window popped up and
opened to yotube.com (a fake youtube copy that harvests peoples phone
numbers).

Her laptop runs Windows 7 and she is a conscientious antivirus (F-
Secure) user, as she makes sure it is kept up-to-date as far as the AV
definitions etc are concerned.

Her IE 8 has the automatic popup protection enabled and at the time my
friend was visiting a site like New York Times or something like that.

So I wanted to ask, what can cause this behavior with IE 8? Could it
be triggered from some advert (or 3rd party Javascript) running on a
legitimate website?

If it was some rogue code running on her laptop I would expect this
kind of behavior surfacing more often...or am I wrong?

Best regards,

Jarmo