Public/ Private IP addresses

General Question:
I work in a small office with about a dozan users. Our network setup includes NT 4.0 server and a win 2000 server. Our new ISP provider gave me a cluster or range of IP addresses. To make my network work I use these addresses for our network but was told I should not for security reasons. How do you setup a network using internal Class C (private IPs) and still have access to the public IPs used for internet access for my users using XP desktop??