Internet sharing and security

Hello everybody
I have a small office contents from one server (win2003) with internet line (Leased Line) and public IP and 18 PC’s as workgroup we work in web hosting so we need full internet access (ftp, messengers,.. etc) I used the RAS service (NAT & VPN) and every thing went OK but I found my server is not secure even my system coz I got errors massages and hackers left files for me.
for that could any one please tell me what is the best way to share my internet to clients with security.
and if possible how to manage the bandwidth (splitting( I want to control the bandwidth for the users.