I have found a similar problem reported on Apple's forum https://discussions.apple.com/thread/1850533?start=0&tstart=0
Copying the solved answer here, "Dashboard is essentially a pared-down web browser. Adobe Flash installers usually look for open web browsers before installing and recommend they be shut down prior to install. So, try going into Dashboard and then move your mouse over any open widgets and then hold the option (or alt) key down and a small should appear on the top left hand corner of the widget. Click that to close the widget. Repeat that for all the open widgets and then retry the Flash installer."
Hope it solves you problem.
Thank-you so much. I will try that now!