Widows 7 Back up problem solved!
Back up hasn't completed in a few weeks...Changed windows back-up setting from manual to 'automatic'.... then ran a full scan of the system using Microsoft System Essentials... the scan took forever but identified about 4 items that had been quaranteed... trojans of some sort. I clicked on 'remove' and then restarted the computer, ran back up again and this time...presto it worked! It is certainly helpful to share problems and ideas like this on such forums- much appreciated! Ian Hope, Edmonton AB