The box didn't go away , even after a couple of hours waiting - left it because I was wondering if something was being unpacked in the background. This kind of information is frustratingly difficult for me to get at on a windows machine... Eventually, I had to use Task Manager to kill it. I went through a few iterations of this trying out a few different ways of uninstalling or running the newer installer (even setting different default browsers since the contents of the box turned out to be HTML and I thought it might be a compatibility issue - but computer says "no").
I looked around for solutions & came across the following, which worked for me:
The most pertinent advice was: