Thanks for the help so far !
Checked expiration date as suggested - expiration = never, Updates expiration = 1 Feb 2007, so that looks OK.
Uninstalled using suggested uninstaller, rebooted - still getting Windows Security Centre error message 'Avast antivirus 4.6.763 (VPS 604.4) reports that it may be out of date.'
Since the program has supposedly been uninstalled, how does Windows know the version number???
Reinstalled program still won't update. If I click on 'Details' on the Update (Basic) screen, program stops responding. After restarting, Clicking on 'Update now' for either program or database doesn't seem to do anything.
Any more suggestions? TIA