Have a look at NOD32 (this link is for the UK site but you can switch to one local to you if necessary);
http://www.eset.co.uk/ I'm only a Home user so I don't have direct experience of using NOD in a business environment, but it has an excellent reputation!
Eset's definition of Small to Medium Business appears to only go to 30 workstations, so you may have to look into the Enterprise edition.