if the vanishing tags contained words like "ads" or "advert" or "banner" or "phpadsnew" anywhere in the path or filename
<A href="http://www.anydomain.com/"><IMG height=90 alt="Any Domain" src="images/www.jpg" width=728> </A>
Then it was almost surely an adblocker issue.. none of that stuff about active-x or browser hijacks sounds logical to me.
If Urb saw this thread i bet he would bet his balls "Norton Personal Firewall" was involved.. or something very similar.