I have just put " after windows 8 update internet explorer and norton toolbar not working" into a Google search, and a number of possible ideas or solutions have appeared. Perhaps worth a look there, to see if any fits the bill, while waiting for someone else to respond?.
Thanks for that suggestion, spuds. I tried your wording in a Google search and, as you predicted, received a number of suggested solutions. Unfortunately all of them suggested tinkering with IE10 in ways that are already set in my version.
Many thanks again Jock1e and to Woolwell for that explanation.
I do wish that Windows updates wouldn't muck about with my settings. Pretty well every time I get an update I find that something has gone awry. Fortunately I have a rough idea of how to re-apply most of them. But it's ruddy irritating!