Okay. Problem appears to be solved. Thanks, zilog for leading me in the right direction by suggesting the vps was being truncated. As I mentioned, the corrupted updates started occurring when I installed Privoxy, which is a proxy server with built-in adblock filtering. I use Google Chrome as my primary browser. In the Chrome proxy settings, I opted to apply the changes system-wide...This modified the gconf settings, applying the new proxy setting to package installs (and probably avast! vps updates). As soon as I changed system-wide settings back to direct-connect (and rebooted), the avast! update problems went away.