It should be fairly straightforward. All computers, your server and any router providing internet access need to be in the same Subnet (IP address range).
Home networks tend to use 192.168.x.x addresses with the Subnet Mask address of 255.255.255.0 which provides a maximum of 254 available addresses. For example 192.168.1.1 to 192.168.1.254 or 192.168.2.1 to 192.168.2.254
For larger networks, and commercial applications, the address range 10.0.0.0 to 10.255.255.255 with Subnet Mask 255.0.0.0 can be used.
To allocate addresses in XP, open the Network Connections folder, right click the network connection ("Local Area Connection" or "Wireless Network Connection") and select Properties. Highlight the entry for "Internet Protocol (TCP/IP)" and click the Properties button. You need to type in the entries for the IP address and Subnet Mask. If the computer is going to access the internet, additionally the Default Gateway address (usually the Router's IP address) and DNS Server addresses need to be entered. The Router's IP address can usually also be used as a DNS Server address.
In Windows 98, right click "Network Neighborhood" and select Properties. In 98, each network adapter in the box that opens has its own TCP/IP entry. Highlight the entry for the network adapter you're using and click the Properties button. In the IP address tab, enter the IP address and Subnet Mask address. In the DNS Configuration tab enter the Host name (the PC's name in the network), type in the DNS address(es) and click Add. In the Gateway tab, type in the Default Gateway address (Router's IP address) and click Add. Click "OK" through the open boxes and then reboot the PC. You *MAY* need to insert the Windows 98 CD to complete the procedure.
This is the first time I've actually set up fixed addressing in Windows 98SE, but it did work OK! XP by comparison is much simpler.