It seems you added the clamav user by hand or some other means, and now the .postinst script is complaining about clamav already existing. Try removing the 'clamav' user and running "apt-get -f install" again.
** Changed in: clamav (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Server Team, which is subscribed to clamav in ubuntu. https://bugs.launchpad.net/bugs/697027 Title: clamav-base upgrade failed on Ubuntu 10.4 -- Ubuntu-server-bugs mailing list Ubuntu-server-bugs@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-server-bugs