Hi,
You need to install some software on your Win98 machines so it'll work on an XP network. XP should create a 'network disk' that you can take to your other (non-XP) machines and install.
Here's what Microsoft Help (XP) has to say about it:
Adding a new computer to an existing Windows network If you have an existing Windows network, you may want to add a new Windows XP Home Edition or Windows XP Professional computer.
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
Your home or small office network could have a combination of different Windows operating systems such as Windows 98, Windows 2000 Professional, or Windows Millennium Edition. In addition, you may have Internet Connection Sharing (ICS) or another sharing service, such as a hardware or software gateway, enabled on one of the computers in your home or small office network.
When adding a Windows XP Home Edition or Windows XP Professional computer to your home or small office network, you may want to consider where in the network your computer is going to be. The following questions provide important things to keep in mind:
Am I going to make the new Windows XP computer my ICS host computer and share its Internet connection with the rest of my home or small office network?
Do I want to protect my home or small office network from intrusions and attacks from the Internet? If you are sharing one Internet connection with the rest of your network, you may want to make the Windows XP Home Edition or Windows XP Professional computer your ICS host computer to take advantage of Internet Connection Firewall, which is a new feature included in Windows XP.
Do I have a mixed network environment? If your Windows XP computer has different types of network adapters installed, such as Ethernet, home phoneline network adapter (HPNA), or wireless, you can use the new network bridging feature in Windows XP to create a seamless network.
When you have your new computer set up and physically connected to your network, it is recommended that you run the Windows XP Network Setup Wizard on your new computer. Using the wizard automatically configures the computer so it can participate in the network. If you are making your Windows XP the ICS host computer, during the Network Setup Wizard, create a floppy disk to run the Network Setup Wizard on the Windows 98 or Windows Millennium Edition computers on your network.
Notes
To start the Network Setup Wizard, click Start, point to Settings, click Control Panel, and then double-click Network Connections. Under Common Tasks, click Network Setup Wizard.
If you are making your new Windows XP computer your ICS host computer and want to enable ICS on it, first disable ICS on the computer using Windows 98 Second Edition, Windows Millennium Edition, or Windows 2000. For more information, click Related Topics.
If you are making your Windows XP computer your ICS host computer, run the Network Setup Wizard on the Windows XP computer first. Then, using the floppy disk, run the Network Setup Wizard on the rest of the computers on the network. For more information, see Configure other computers on your network.
The Network Setup Wizard can only be run on computers running Windows XP Home Edition, Windows XP Professional, Windows 98, Windows 98 Second Edition, and Windows Millennium Edition.
Just press F1 on your XP machine and type in Windows 98 - there's a whole lot of other links in there that will help you with your installation.