If your computer has a wireless card installed, you can use it to send a signal to other computers like a router. Every computer has an application for setting up wireless networks on the system, and any computer can be configured to accept that wireless signal. You can also create a security key for the network to allow only authorized users--an essential function if you don't want other people stealing your signal and possibly spying on your network.
Make sure your computer has a wireless card for Wi-Fi capability. If it doesn't, install a wireless card in an ISP slot on the back of your computer. Connect the modem to the computer and confirm that your Internet access is working.
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Go to the control panel and select the "Network" option. In Vista, select "Network and Sharing Center" followed by "Set Up a New Connection" in the "Tasks" menu. Click on "Set up a Wireless Ad-Hoc Network."
If you use an XP operating system, go to the "Network Connections" option. Right-click "Wireless Network Connection," and select "Properties." In the "Wireless Networks" tab, check the "Use Windows to Configure..." box and click "Add" under "Preferred networks."
Create a name and security key for your new network. In XP, choose "Shared" as the "Network Authentication," and "WEP" for the "Data Encryption." In the "Advanced" option, choose "Computer to Computer Networks Only."
Complete the wireless setup. In Vista, click "Turn on the Internet Connection Sharing." In XP, click "OK," then right-click "Local Area Connection" and select "Properties." Go to the "Advanced" tab and check the box for allowing other network users to connect to the network.
Turn on another computer (with a wireless card) and open the network sharing center. Click the option for viewing or managing wireless networks, and select your new computer network (you may have to "Add" the network before it appears). Input the network's security key to access the wireless network.