First check the computer that is going to be upstairs, Suggest you put the Router\Modem on that Connected by Ethernet Cable. Check there is a Lan Socket on the Computer. The computer downstairs I take it is a Laptop, check it is Wi-Fi if Not you need a PCMCIA Wireless Card or a USB wireless Dongle to connect to the Router. You can just have one connected to the Net or both at the same time make no difference. You do not need to Network them, Only if you want to Transfer Files and Use Printers. Hardware This is a good one, I use it click here
You need a wireless router; this will be used instead of your current router. Both PCs need wireless "cards" - they may already have them installed if they are newish; mine were not so I needed one in each. Set-up CDs will come with the router. Mine is a BT Voyager 2100; I followed the instructions and everything worked fine from the start. Incidentally, I bought the kit from BT because their website gave the clearest explanation of that I needed and I could buy it all in one pack; but that was a couple of years ago. Last week I was in Currys and picked up a helpful booklet there to pass on to a friend wanting to do the same as you. He won't necessarily buy the parts there, but the info and diagrams make it all very clear to understand.
My daughter bought a cable router from linksys last week and I had some difficulty setting it up. I phoned their free technical helpline and was up and running in less than 5 minutes which included intructions for setting up the wireless connection to the laptop and a wired connection to the desktop. The Linksys call centre is in the Philipines and their expertise and helpfulness was exceptional.
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