True, but a smart advertiser doesn’t try to install something, and uses what the user already has. I know that I had a computer without Flash, and I was never asked to install Flash to see an ad (that I know was Flash on my other computer). It was just replaced with an animated GIF.
The only ads I’ve seen that try to install stuff were on “questionable” sites, and what they wanted to install was malware.
So, I would go so far as to say you should NEVER try to install an ActiveX control directly. The user should see a banner that asks them to install and lets them click it. Seeing a warning come out of the blue will scare almost anyone.
This may in fact be what happened here - IE threw up a warning banner at the top of the page saying that an activex couldn’t be installed, but that was due to policy imposed on this machine (at work) - I guess if it had been my own PC, I’d have seen a banner asking me to confirm if it was OK to install it.