In most of the cases the main reason that this error pops-up is the registry installation data of the software. It seems that the registry have been corrupted and the update, although it is downloading correctly, doesn't know where the main program installation is and then it gives you the error you specified. A quick resolution to this problem is to download the setup package of the antivirus and then reinstall it using the Repair option during the installation then perform the update as necessary.
