For those who want an easy to setup anti virus I highly recommend the use of clamassasin combined with ClamAV and Procmail.
First you'll need to setup your MTA and Procmail as usual, then get clamassassin from [http://drivel.com/clamassassin/].
Download the package supplied and extract it, and follow the README file.
formail is part of Procmail remember, not a separate package. Also mktemp is most likely already on your system , to test try it on the command line, if you don't get a command not found error then you should be OK.
You should be fine following the README file included. Remember to check the paths in the CONFIGURATION section of the clamassassin script to those of formail, mktemp and clamscan.
 Some systems (Slackware, for instance) do not install this by default.