If you're running CustomBuild 2.0 exim.conf changes are not required, as it uses .include_if_exist options in the exim.conf. Skip to Step 3.

1) Edit your /etc/exim.conf, and add the following before "primary_hostname =":

av_scanner = clamd: 3310

*** Skip this step if you are running CustomBuild 2.0, as it adds this for you with /etc/exim.clamav.load.conf.

2) While still editing your exim.conf, find "check_message:", and add this code below it:

deny message = This message contains malformed MIME ($demime_reason)
demime = *
condition = ${if >{$demime_errorlevel}{2}{1}{0}}
deny message = This message contains a virus or other harmful content ($malware_name)
demime = *
malware = */defer_ok
deny message = This message contains an attachment of a type which we do not accept (.$found_extension)
demime = bat:com:pif:prf:scr:vbs
warn message = X-Antivirus-Scanner: Clean mail though you should still use an Antivirus

3) Restart exim:
service exim restart