I googled and read a number of ways of solving the MSVCR80.dll missing - none of them worked.
Finally, I uninstalled everything from my installed Programs which indicated it was owned by APPLE:
*Itunes
*Apple Software Update
*Apple Mobile Device Support
*Bonjour
*Apple Application Support
*Icloud
*Quicktime
*Safari
*MobileMe Control Panel
I am wondering whether the culprit might be Bonjour - in attempting to uninstall I received a message saying that I was unable to do this as the program MSVCR80.dll needed to be closed. I was given the option of ignoring this message which fortunately I did and Bonjour was uninstalled.
After uninstalling all APPLE programs, I shut down my computer. Upon restarting, I was successful in reinstalling Itunes.
This is not the first time Apple has succeeded in stuffing up the computers of millions of people with one of their software updates. This is very disappointing and I hope that in future the Apple technicians pay careful attention to what it is they are doing before they release updates to consumers.