http://giytdyrio.info/pcdiag/1/alert1.php didn't get blocked by Avast, but it's a fake blue screen of death page that I couldn't get out of by any method other than restarting the computer.
Scary stuff. Wanted me to call a number or dire things would happen to my data, etc.
I looked around for somewhere to report this, but couldn't find anything. So I'm posting the link here and hope if it's the wrong place that someone with the power to move it to a better one will do so.
I didn't think the ad I clicked on looked particularly dangerous.... (an old story, right?)