problem with NAT in a windows 2000 domain

i have "hussain.com" windows 2000 domain. "arslan.hussain.com" is the only domain controler of that domain.I am running DHCP and DNS both on that domain controler. I also impliment NAT on that domain controler. But at client side when client want to access the internet they got an error message that con't find the DNS server. Even all the settings in the clients and DC are right.
Please give me the solution how can my clients can resolve the internet addresses. More i am using dialup modem to connect my domain controler to the internet.