I have been looking into this and have not done it yet so am not an expert.
You would need to purchase 1 wireless router, 1 wireless pci card and one pci network adaptor.
The computer that is beside your phone jack would need the pci network adaptor installed and would connect to the wireless router using a network patch cable. The PC away from the phone adaptor would need to have the wireless pci card installed to communicate with the router.
You do not neccessarily have to buy a router with ADSL modem built in, you may be able to connect your exisiting modem to the router.
To connect with the PC with the wireless card in, you would need only the router and modem to be powered.
When looking at wireless equipment, you will also see a 'b' standard and 'g' standard of equipment. The 'g' standard is 5 times quicker, and is good if you intend to transfer files between computers, but if it is only for ADSL net access the 'b' standard is more than fast enough, and cheaper.
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