I run the same as oj88.

pfSense on the network layer. This acts as my firewall, my web caching via Squid, my QoS, and routing functions (load balancing and failover).

For clients, I typically use a mix of Windows Security Essentials and Avast! anti-virus depending on how secure I need them to be. Yun problema with Avast! is that there's a known security flaw with Windows 7.. I kinda forgot what it was but it required me to do a fresh reinstall on the system since I just can't find a fix for it. It has something to do with drivers that is associated with Avast! -- yun tipong kahit safe mode hindi ako makapasok (it just hangs in one .dll)

Windows Security Essentials is essentially headache-free. I really love it for the low profile and non intrusive software layer. As a testament to it, both of my computer systems (home and work) uses WSE.