Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.
Then try a manual program update again.
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If that gives any errors it may be best to try a clean install of avast 7.0.1456, more on that depending on the outcome of the Repair and manual update attempt.