I never got any before either, 2 times now..

yes it will all of a sudden pop up.
For me it minimizes the browser window and a warning notice pops on the screen. Something to the effect of " You may have a virus, get this Anti Virus 2009 and it will clean your system "
{I didn't read the whole thing cause I wanted to get it shut down right away}

since it was mentioned here before I knew to not click on it anywhere at all or it would activate it.