Protection in a network must be done on more than one level. We all need the internet and to protect intrusion from the internet, a firewall is needed to block such attempts. A properly set up ADSL router can be all that is required to block access to your network from the internet. If a connection is made with something like a diginet line, a proper firewall is required. We use linux PC's as firewalls.
Protection on mail servers is discussed on the mail server page.
We protect our clients workstations with anti-virus software. The software we use mostly is from Eset.
It is very nice and convenient for us to be able to log into the server of a client to do some maintenance work. This is a problem because it leaves a possible entry point for a hacker to gain access. We protect the client with VPN and good passwords.