The 403 error 'HTTP Error 403 - Forbidden' is somewhat strange as access shouldn't be forbidden as the url for the update servers originate from within the avast servers.def file generated/updated by avast. And the manual update which works uses the same servers.def file.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
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