In the Postfix guide, Amavis and ClamAV are installed from Debian stable.
Read it here
Debian Sarge is just far too old. Having played with the new Amavis-new 2.4.2, it's changed significantly in the Debian package where /etc/amavis/amavisd.conf has been completely replaced, so we are going to leave amavis-new alone and just leave it from Debian sarge stable. However ClamAV is a little outdated so we are going to take a fresh copy from testing.