Anti-Hacking Software

We had a 64K leased line from jeddah to dubai and all the desktop has internet connection trough this line from Dubai ISP etsilat. I am afraid our users PC which are running on Windows Workstation 4.0 could be hacked, can anyone suggest me who has tested and used any antihacker Software, freeware or shareware from internet or any vendor,I am really serious to protect all these PC’s which are exposed to internet.
thanks awaiting your answers