A great on premise business solution - Microsoft Windows Server 2012 Essentials.
The Cloud based version MS Office - Office 365 - includes excellent email and collaboration
Our recommended antivirus for small offices is Trend Micro, who do both server based and hosted solutions - the-it-service is a Trend partner.
Microsoft's own free security suite is no slouch - Microsoft Security Essentials.
Check the effectivness of your firewall using Shields Up! from Gibson Research.