This is dangerous b'coz if the user plug in his pendrive/floppy/CD from external source, the virus will easily penetrates thru his PC and eventually to all workstations in ur firm. U can always advice ur collegue to scan his media before plug into his workstation. But the chance of getting infected is there coz human error (e.g. ur collegue(s) may forget ur advise.) So, install free (valid for commercial use) AV in all workstations.

My 1st option for corporate and INTEGRATED protection will be Kaspersky Corporate Suite. Should ur budget is low, try ZoneAlarm Internet Security Suite Small Business Edition.

But, if possible, try all network security/protection available in the market first before u decide. This is to ensure network compatibility and system operationals/functions do not clogged by the AV product(s).