[update]
Several of my PC related problems seem to have been resolved including the one on this thread. For example, my desktop has now been running for several days without being rebooted and with no obvious problems. It had been crashing at least once a day and sometimes several and would rarely wake up correctly. I had also been experiencing weird problems with Google and email.
All these seem to have been resolved I'm pleased to report.
So a big
THANK YOU to everyone who made valuable suggestions.
Three things seem to have contributed to this happy state of affairs:
1. A careful clean install of the latest nVidia GFX card drivers (several of you made useful suggestions there).
2. Moving all my old mail on the server out of the server's Inbox (it was VanB's suggestion about a possible corrupt email that led me to try that again). I still don't know for certain whether there is a corrupt email but moving all of them out of the way has stopped the problem.
3. Resetting the browser settings on Internet Explorer so now Google Street view works smoothly again. (Thanks to MrValentine.)
Thanks again everyone.
[/update]