Best Antivirus for Public LAN (cable net)

I need a antivirus for Win XP which is best for network security plus best antivirus im on a public network (LAN cable internet) with internet connection with more then 550 computers and most of them have virus. or if someone recomends a good antivirus and firewall ??? i dun know much about it so will someone help me about this . i hope u pplz understand my issue.