I don't run Windows servers, so I can't tell you where the IP address is set, but it may be in Properties for the network card (Control Panel/Network). If there's a specific IP address entered (there probably is), you'll have to set it to the subnet the router is on. Give us the make and model of the router and someone can give you that information, or go to http://screenshots.portforward.com/#A and look it up. (The first 3 numbers of the server's IP have to be the same as the first 3 numbers of the router's IP, and the last number of the server's IP has to be outside the DHCP range of the router AND an address not used by any other computer in the network.)
A subnet mask of 255.255.255.0 will probably work.
For the DNS addresses, you can give it the router's address (one of the functions of a router is to route DNS requests).
The Gateway address is the router's address.
BTW, you don't need to be running a server OS to run Wordpress. Windows 7 running xampp will work just fine. However, if you want people outside your network to be able to get to your Wordpress blog, whether you're running Windows Server 2003 or Windows 7 (or any other OS), you'll have to forward port 80 - in your router - to the computer that Wordpress is running on. AND hope that your ISP isn;t blocking incoming port 80 requests. If they are, you can run your web server on a different port (81, 8000 and 8080 are common work-arounds). Then your users have to connect to that port or you can use an IP translation service like DynDNS to translate the incoming port 80 requests to the port you're using. (DynDNS has a free service.)