Hi, I have 3 computers all 3 are windows xp,I have the main PC connected to a modem/router, the second PC is connected to the router from a network card/usb wireless card.(netgear) . Now I want to connect my third PC which is in another room(down stairs) the other 2pc's are in different rooms upstairs. How can I connect the third PC, do I have to purchase another wireless network card? Also I'm not sure this third PC has a ethernet card installed, the back of the tower shows a ethernet port but I'm not sure. Any advice would be welcomed. Thank you!:)

Using a wireless card between upstairs and downstairs computers has all kinds of problems unless they are fairly close. The best would be to connect the downstairs PC to the router by cable. You will need the appropriate length of Cat5e UTP cable (straight). More problematic will be working out how/where to run the cable.

To work out if your 3rd PC has an ethernet card go to System under Control Panel and click on Hardware and then Device Manager. You're looking to see a network adapter listing.

All new PCs have a wired Ethernet port. But if you don't want to run a wire, then you will need to get another wireless card for it.