Server End Spam filter needed

Heya chaps and chapesses

Wondering if anyone out there has any products they recommend for cheap spam filtering on a server, I have a mail server that I started for myself and now is run for not only me and my children but an increasing number of friends and relations.

It is an NT machine with firewall protection, AV protection etc running the www.youngzsoft.com cmail server. I like this server software (I can understand and manage it with little or no serious thought which is good as it isnt a business, just a cost to me)

If anyone has any thoughts as to how i would love to hear them, some of my users come into the seriously ancient and crumbly category others are as young as 8 so i need to make it happen for them without them having to think. We also seem to have a mixture of Mac and PC users.

Thanks

Marcus