Originally my Windows XP Professional computer was directly connected to the ADSL modem, and received an IP address in the range 10.1.1.x. Now, the computer is connected to a SMC 7004ABR NAT router (which sits between the computer and ADSL modem). However, the Windows XP computer will not accept the new IP address (in the range 192.168.2.x) offered by the SMC 7004ABR. The SMC 7004ABR is configured as a DHCP server and I have disabled the DHCP server on the ADSL modem. A Windows 98 SE computer is also connected to the SMC 7004ABR and has been configured with a permanent IP address, which works with no problems. How can I assign a permanent IP address to the Windows XP computer? I have been unable to find the necessary settings.

From the description you provided, it seems either the Windows XP computer has been configured with a static (permanent) IP address in the range 10.1.1.x, or the IP address has been dynamically assigned but has not been renewed when connected to the new SMC 7004ABR router (causing the IP address to still remain as a 10.1.1.x address). The latter explanation seems strange because Windows should automatically request a new IP address from the DHCP server when connected to a new network. So, we will start with the more likely explanation that the IP address has been statically defined. This will also answer your question on how to assign a static IP address in Windows XP. Older versions of Windows, such as Windows 98, allow you to specify the TCP/IP configuration for internet connections through the ‘Network’ control panel, as you probably discovered when configuring your Windows 98 computer. However, Windows XP has put these configurations in a more intuitive location, allowing you to configure the TCP/IP settings for a connection through the properties for that connection. To view all the network connections, go to the ‘Start’ menu > ‘Control Panel’ and open ‘Network Connections’ (you may need to first open the ‘Network and Internet Connections’ category). Because your computer is connected to the ADSL router through the network port on the computer, we need to configure the Local Area Connection. Right-click on the icon for this connection and select ‘Properties’. In the window that appears, click the ‘General’ tab and under ‘This connection uses the following items’ highlight ‘Internet Protocol (TCP/IP)’ and click ‘Properties’. In the window that appears, you can configure the IP configuration for your connection. Once you are finished, click OK then click OK again to save and close the windows.

Taking your question a little further, we will now address the situation where you find even though the IP address was not statically defined, it still remains in the 10.1.1.x range. Furthermore, you wish to have the IP address dynamically assigned by the DHCP server running on the SMC 7004ABR router. In this case, it appears the IP address was not renewed when changing networks (i.e. from the ADSL modem, to the SMC 7004ABR router). So, the computer is still assigned the old 10.1.1.x IP address from the ADSL router. In this case, we can force a renewal of the IP address. Go to the ‘Start’ menu > ‘Run’ and type ‘cmd’ (without the quotes) and click OK. A command prompt window should appear. At the command prompt, type the following command:
ipconfig /renew
This will ‘renew’ all network adaptors and in the process attempt to obtain a new IP address from the DHCP server running on the network (in this case, is running on the SMC 7004ABR router).

As a rule of thumb, if you are running a DHCP server (so you can easily connect other computers to the network), but also have some computers connected with static IP addresses, you should reserve these static IP addresses on the DHCP server so the server does not attempt to assign these addresses to other computers. You should be able to do this through the router interface.