How do I network three diff OS and connect the Internet?

I have 3 different computers in my home that use 3 different operating systems, only one is able to access the Internet now because I have no idea how to connect all three (Network) to the same router from different places in the house. Can someone please give me an idea how to get all my computers online? I realize I need wireless set up but I don't know exactly what items each computer will need installed in order to be connected to the network. The current Internet capable is a Dell Inspiron 530 desktop with a Vista os, #2 is a Dell Dimension 8200 desktop with XP os and #3 is a Dell Inspiron N5050 laptop with Windows 7 os.

The laptop does have a WLAN card installed already. I would like to be able to take the laptop with me when I travel to communicate with my clientel etc. I just don't know much about WiFi.