|This is W95/W98 forum - is one of the machines using Win9x?|
Whatever, if you have a "wireless router/modem" you can get a "Wireless Adapter" which plugs into the USB socket on the machine which has no Ethernet card. It looks a bit like an overgrown Flash Drive and serves as a transmitter/receiver once you have installed the software.
To prove the point, I have XP cabled to a wireless router/modem which serves both my Vista laptop and my old W98SE machine. The laptop has inbuilt wireless facilities and my W98SE uses a USB Wireless Adapter, as given above.
All you have to do is make sure any machine you add has the same wireless settings as the one with the wireless router - particularly security.
If you don't have a "wireless" router/modem then I'm not sure how you go about it - someone else might suggest something.
some other bloke...