On all my Home Windows biased PC's I use - Avast Professional Edition:
1 Dell XPS-400 - XP Media Center Edition
1 Toshiba Laptop - XP Professional
1 Older Toshiba Laptop - XP Professional
1 "White-Box" - XP Professional
I use Symantec EndPoint Protection 11 - I use on my Servers:
1 PowerEdge Server - Windows 2003 Standard Edition
1 IBM xSeries 306 - Windows 2000 Advance Server Edition
Not using any Anti Virus Protection:
1 Mac Mini - No Anti Virus Protection
1 Mac Dual G4 - No Anti Virus Protection