[newbie] Updating a server

[Automated Fax System/System de Fax Automatique]

I've got an old slackware 6 server running Dhcp, mail, Web and firewall duty
here and I want to know if there's a wa to update it to the latest version
WITHOUT losing any of the configuration. I didn't build the server so i
don't want to screw anything up as it acts asa mail server for about 30
users. any suggestions ?

On the same note, Is there any way to filter mail so as not to let
attachements of a certain size through and to check for virus (i think the
last one might be asking too much) ?s