Follow these steps to install the NAT Service..its easy!
Click Start > Programs > Administrative Tools > RRAS. When the RRAS window opens, right-click on the name of your server and select Configure and Enable Routing and Remote Access. After the service is installed and running, you are ready to install NAT.In the RRAS window, right-click on IP Routing and select General, then select New Routing Protocol followed by Network Address Translation. You will have to add at least two network interfaces. As you add each interface you can choose whether it is "public" (attached to the Internet) or "private" (attached to the internal network).
Now we can configure some additional options for NAT. Right-click Network Address Translation and select Properties. The NAT Properties window will appear with four tabs to choose from: General, Translation, Address Assignment, and Name Resolution.Logging properties are set under the General tab. There are four fairly self-explanatory types to choose from.You can set the timeout values for TCP and UDP sessions under the Translation tab. The default value is 1440 minutes, or 24 hours for TCP, whereas the default for UDP sessions (which are connectionless) is 1 minute.The aptly named Address Assignment tab is the place where you can choose whether you'd like NAT to automatically assign IP addresses by using DHCP or not. If you check the box indicating you want this, a mini-DHCP server called the "DHCP Allocator" will hand out addresses in the 192.168.0.x range with a default 24-bit mask.
Under the Name Resolution tab on NAT Properties, you can have NAT act as a DNS or WINS proxy agent for your private clients. The WINS proxy doesn't have all the functionality of the full-blown W2K WINS Proxy Agent. NAT clients believe that the WINS proxy is their WINS server (the address is handed out by the DHCP allocator as a WINS server), but the WINS proxy simply drops name registrations made by private clients. You will need to have an alternate method of resolving names within your private network (LMHOSTS) if it is divided into broadcast segments. I simply prefer to avoid the hassle and use a standard DHCP server and have it configure clients with the address of a standard WINS server (the DHCP server must specify the IP address of the NAT server as the default gateway). If you want to use your own DHCP server, make sure the "Automatically assign IP addresses by using DHCP" checkbox is cleared – this turns of NAT's DHCP Allocator.Configuring Your NAT Clients:Okay, now the easy part. Open the Network Applet on your client machines and enter the IP address of your NAT server as the gateway in the client's TCP/IP properties (if you are using manual addressing). If you are using the DHCP Allocator, configure the client machine for DHCP and it will pull down the necessary settings.
Or you can manually assign IP addresses and set the NAT IP as the Gateway on the clients. Thats it..your done !