I am going to go step by step.
1) Connect you Linksys directly to the Cable modem
2) Connect both you PC and your Server to the Linksys
3) Use Internet explorer to go into you linksys router and look at you
status. The status should tell you what the two DNS servers are for the
internet. You will need this later to setup forwards on your 2003 Domain
controller.
4) Install 2003 Server with a static IP address I think the router uses
192.168.1.1 so set the server to 192.168.1.2 use 192.168.1.1 as the default
gateway Set your DNS server to 192.168.1.2 (yes that is the same IP as your
DC which is correct)
(these instructions are from a 2000 server Domain controller environment
but should work with 2003)
5) Run DC Promo or Active Directory wizard. When you choose a domain name
use name.local where Name is any name you chose and .local. The .local
signifies that it is not going to be used as an internet domain.
6) run the MMC for DNS after AD has finished installing. If I Recall
correctly you should only see a forward and reverse lookup zone for you
domain. if you see something that is simple a period for the name you need
to delete that so that you can setup forwarders. I believe the period
signifies a root domain which you do not want!
7) In the DNS MMC Right click on server name and go to properties from there
click on forwarders. enable forwarders and put in the DNS address from your
ISP.

open your Internet explorer to check if it works. If you can browse the
internet continue
9) load DHCP and disable DHCP on the router.
10) create a DHCP Scope for 192.168.1.1 subnet mask is 255.255.255.0
11) Exclude 192.168.1.1 through 192.168.1.5 (I would do this so you could
add other static devices such as printers)
12) set the DHCP scope options i.e. router (192.168.1.1) DNS (192.168.1.2)
13) Authorize DHCP server
13) reboot you XP machine and see if it gets an IP address and can browse
the internet.
At this point if all is working you should be able to join you XP box to the
domain
These steps will work but there are many details I have left out. If you do
the research to understand all of this you will be able to complete the
task. It will also help you with server test which I expect you are also
looking to take. Hope this helps
Rick