Unless you are using a ROUTER, only one PC will get an IP address from the ISP and have internet access. When you install a router, it gets the single IP address from the ISP and allocates IP addresses in one of the ranges reserved for Local Area Networks to computers connected to it, most commonly a 192.168.x.x address. It then "routes" the internet traffic to the appropriate computer in the network.
If you are not using a router, you can connect one PC to the internet, network the two computers and then enable "Internet Connection Sharing" on the actual internet connection (modem connection) in the "host" PC.
To enable “Internet Connection Sharing” in Windows XP, open the Network Connections folder, right click the actual internet connection (modem connection), select Properties and then the Advanced tab. Tick the box “Allow other network users to connect through this computer’s Internet connection”.
When you enable "Internet Connection Sharing" on the Internet connection, it automatically configures the network adapter used for your "Home Network Connection" with the IP address 192.168.0.1 and subnet mask 255.255.255.0. If you have more than one adapter available for this purpose, e.g. both a "Local Area Connection" and a "Wireless Network Connection", you will have a drop down list from which to select the appropriate adapter. The "Client" PC's Local Area Connection should be set to get its IP address automatically, which it does by DHCP from the "Host" PC. Also, if you run the Zone Alarm firewall in your "Host" PC, the Internet Zone Security level must be reduced from High to Medium or "ICS" traffic will be blocked.