Open Control Panel, and in Classic view click on the Wireless Network Connection wizard. Click the option for an ad hoc network and then answer the questions. This creates the network in XP. Once this is done you can connect to any device that has a wireless adapter. For more information open Help and Support and search for ad hoc network.
Please note that every ad hoc network is deleted when all of the devices using it have disconnected unless you choose to make it a permanent network when you create it. To create a new ad hoc network simply run the wizard again.
Ad hoc networks are always wireless and utilize Internet Connection Sharing. If you share your connection this way ICS will be disabled if you disconnect from the ad hoc network, you create a new ad hoc network without disconnecting from the old one first, or you log off and then back on without disconnecting from the ad hoc network. Keep this limitations in mind when using ad hoc network so you don't get caught in weird error situations with cause you to scratch your head.
One last cravet, if you let somebody share your system with fast user switching you share the ad hoc network with them too. So, be sure you want them accessing your network shares.
Edit: How odd, when I first ran the wizard setting up my Wireless Network I was given an option to create three different types of Wireless networks, and ad hoc was one of them. However, I re-ran the the wizard (you can go through the pages without actually finishing) and I wasn't given any option except either manual or USB setup; clicking manual took me past all of the normal pages and instructed me to print out my settings and then manually enter them into computers I am adding to the network. So, I apologize for misleading you with this response. Try this link:
http://www.microsoft.com/windowsxp/using/networking/setup/adhoc.mspx
