Antivirus

What's the best antivirus i should use in my 10 computer network?
also how i prevent internet attacks?

i installed norton internet security and all my computers slow down

also... why i installed pc cillin in 3 computers and internet security
in other 3 the problem is that the 3 that have internet security can't
see the 3 that have pc cillin and the same problem in the opposite
way.
Thanks


Marco Castro
(E-Mail Removed)