You want to set up an ad hoc wireless network, in which you don't need a router to connect PCs they can connect to one another directly.
When you create an ad hoc network, WiFi-enabled PCs within range of each other communicate with each other in a peer-to-peer fashion. To create an ad hoc wireless network, first install wireless cards in all the computers to be networked, and then do the following:
On one of the PCs that you want to be part of the ad hoc network, click the wireless icon in the Notification Area, choose Properties, and click the Wireless Networks tab.
Click the Add button to get to the screen shown in Figure 14-11. In the Network name (SSID) box, fill in your network name. Check the box next to This is a computer-to-computer (ad hoc) network; wireless access points are not used. Other wirelessly equipped PCs will connect using the network name you've just chosen. Don't use encryption or authentication at this point. If you want to use it, you can set it up later, following the steps in Recipe 14.7. Click OK.
Figure 14-11. Naming your ad hoc network
The ad hoc network you just set up will now appear in the Preferred networks portion of the Wireless Networks tab. It will be displayed with a PC card icon next to it, to denote that it is an ad hoc network. (Networks that use access point show up with an icon of a small transmitter next to them.) It will also have a small red X on the icon to show that there are no current ad hoc network connections.
On another computer that you want to be part of the ad hoc network, click the wireless network icon. Click View Wireless Networks. If the ad hoc network shows up there, highlight it and click the Connect button. If it doesn't show up, click Refresh network list. When the network shows up, highlight it and click Connect.
Follow the same steps for any other PC you want to add to the ad hoc network.
The first PC that you configure for the ad hoc network serves as the network's host. So if you're planning to use the network as a way to share a single Internet connection, make sure that you choose the one with the highest-speed Internet connection as the host.
Wireless routers are so inexpensive that there is no economic reason for setting up an ad hoc network. Ad hoc networks are best used when you need to transfer files between two PCs and there is no router available. Ad hoc networking without security puts your PCs at considerable risk, because providing file sharing over ad hoc opens your file system to anyone who can get a signal. Be sure to disable ad hoc mode as soon as you are done using it.
For more information about setting up an ad hoc network, see http://www.microsoft.com/windowsxp/using/networking/expert/bowman_02april08.mspx.