Several things to look at:
*A true server. Move away from the peer to peer/workgroup environment, and put a proper server in place. This gains you better control of file access, and a central location to store all your files on a computer that's expected to have much higher reliability than a desktop computer, hardware RAID on the drives, better performance, and proper backup. Some good condidates for a small office....Microsoft Foundation Server, for basic functionality. And Microsoft Small Business Server....for a nice server suite of applications for a small business
*Better antivirus....my opinion, based on experience, is McAfee is poor protection. Even the free antivirus from Microsoft, called Microsoft Security Essentials, is better..and free for home users and small businesses of up to 10 PCs. Trend Micro, Eset, Symantec, Avast, ...are other good alternatives.
*Battery backup unit on your server
*A proper daily backup of the server, including an offsite backup strategy, and an end of month backup
*A UTM appliance as your router. UTM = Unified Threat Management. The days of a basic NAT router for a business are gone, UTM appliances give your business network much better protection, they provide features such as additional antivirus scanning of internet traffic, anti spyware scanning, etc....at my clients that I've put in UTM appliances...I have much much MUCH less problems with malware.