It's the Comodo Verification Engine that's causing the problem. This program verifies a website as genuine. I've stopped it loading and there's no problem. Strangely though it still works because it still verifies sites even though the icon no longer appears in the systems tray.
I had noticed something strange earlier: on starting my browser in Sandboxie, a second Comodo Verification Engine icon would appear in the systems tray and would stay there till I closed the sandboxed browser. Now, of course, no such icon appears in my systems tray at any time.
So problem solved and it's probably not worth your time looking into it further, especially given that I am running Windows 2000 and the offending program is probably one not many people use. Still, useful information for anyone coming across a similar glitch in the future.
Many thanks for your interest and for such a cracking little program.