How do I setup a win 2003 server on a shared DSL line.

Depending on the budget you have, I would say the router between the 2 is where you need to focus.  You can connect a router (Router A) between the modem and Comapny A (Just internet company).  Also connect a second router between Router A and Company B.  On the second router you need to just change the subnet mask from 255.255.255.0 to 255.255.0.0.  This will allow both companies to utilize the internet but disallows Company A from connecting to Company B's server.  There are other ways but this would be the simplest to do with a low budget and technical assistance.

Taurance,

The second router was the first choice. What is another way to get the result I'm looking for? Thanks

Another way to do this is to just change the subnet mask manaually on each system that needs to use the Server(and including the server).  To change the subnet you also have to set the IP address and the Defualt gateway which should be the same as what it is atm with DHCP turned on for the workstations.  

Taurance,

Once in TCP/IP Properties for the DNS Server address will I be using the one that the ISP provides? WIll the IP address that was assigned to the workstation from the DHCP get reassigned at some point.

Actually better to use 2 routers in parallel. First scenario Company B can access company A bur not the reverse. If you want to completely isolate, obtain a second IP from your service provider (you may already have a block of 5 if it is a commercial connection) and add a switch in the configuration. This totally isolates both networks, and will allow both to support incoming services such as remote management or VPN's if necessary.

               Internet
                    ||
                Modem
                    ||
                Switch
                    ||
     ===============
     ||                             ||
Router A                Router B
     ||                             ||
A network             B Network

I am sorry Rob is correct, I set the Subnets up backwards.  On the router change the Subnet Mask to be 255.255.0.0  this will give all the other computers that do not have a static IP address the 255.255.0.0 subnet.  

The DNS information would be exactly what it is before you give it a static IP address.  So if you are using a normal linksys home router lets say and you have the 192.168.1.X layout, I would set your ip to 192.168.1.100 => 110 (if you only have 9 pc's and the server).  The Subnet would be 255.255.255.0.  The Default Gateway would be 192.168.1.1 The DNS server would be 192.168.1.1 .

The 192.168.1.1 is the ip address of the router.  If this is just a really simple network you have then all this information is going to come from the Router.

By assigning IP addresses, these addresses are not going to be allowed for the DHCP of the router to give out.  To take this a step further you should also connect to the router and setup these staic IP's there so that if the pc is ever turned off the DCHP doesnt assign that IP to a pc.  When the Static IP computer is turned back on there would be a conflict.  But if you set the IPs not eligible for DHCP on the router you should be fine.  

Lets see if I have this right. On Rob's example Router A has a subnet of 255.255.0.0 and Router B will have a subnet of 255.255.255.0. Both routers are then set up as DHCP. Is this correct?

Actually in my configuration either network can use any IP addressing scheme  (including subnet mask) they like, as they are totally separate networks and cannot "talk', regardless of addressing schemes.

I was assuming you wanted to isolate them.

yes I do want to isolate them. Can I still setup both routers as DHCP enable?

Thanks D3XT. Good luck with it.
Cheers !
--Rob

But is you do not want to add more hardware, going with 1 router could also do the trick using the different Subnet Masks.  It would be like this:

           Internet
                    ||
                Modem
                    ||
               Router
                    ||
     ======================
255.255.255.0              255.255.0.0
     ||                                       ||
Network with        Other company network
Server                                  (DHCP)
(Static IP's)

and if you dont want to use Static IP's you could also do it this way

           Internet
                    ||
                Modem
                    ||
                Router A
          255.255.0.0
                    ||
     ===============
     ||                             ||
Router B            Network 2        
255.255.255.0
    ||                            
Network 1

This would allow you to have DHCP on both networks.

Let me know if this helps.