Verizon has released its 2013 data breach investigations report. In that report it stated that 40% of all breaches were related to malware. Of those breached, the malware was distributed via email.
One of the most important security items for any network is a good email threat detection system. This should include spam detection, virus scanning and the ability to stop phishing emails.
Once this malware is installed on your computer system, the goal is to stay long term to monitor, or become an attack point for someone.
A good UTM or next generation firewall will also help detect and stop malware.