Comment 2 for bug 812806

adduser: O utilizador `clamav' já existe. A sair.

It says that the clamav user already exists. Try purging the clamav-base package, and reinstalling clamav. Open a terminal, and type:

sudo apt-get purge clamav-base

After that you can reinstall clamav. If you did change the clamav configuration, you should back up /etc/clamav/clamd.conf .