Spam and virus filter for mailserver

[Daniela]

Hi!

We have the following situation and don't know what to do:

I have one POP3 account with alias adresses for the network clients, and every 
client configured the mailer not to get mail destined for the other clients.
Now we get so much spam and viruses, and I decided to set up a mail server, 
recieve the mail for all the clients, and the clients get their mail from my 
mail server, keeping their current mail adresses.

The clients run M$ Winblows eXPensive, so I need an antivirus program that 
runs on FreeBSD (should be free). Can anyone recommend a good mail reader 
that can fetch mail as a daemon, stores mail in a format readable by KMail or 
Evolution, and has a console interface (I don't run X often)? It should also 
be somehow able to put the mail in different folders based on the value of 
some headers.

And what mail server should I use? It should be secure and not use up many 
resources (security has the highest priority).

Is this stuff possible at all? Please help us.
Thanks in advance.

Daniela