Assuming your desktop PC has a wireless card, all you must do is enable Internet Connection Sharing.
http://www.practicallynetworked.com/sharing/xp_ics/
If your main PC does
not have a wireless card, you must buy a wireless router to share stuff. My recommendation is the Linksys Wireless G router, which includes a built-in firewall, and is completely configurable. Once you have one, plug the main PC into the router, and then configure any wireless devices to connect to this router. To share the internet, click on the link above.
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