How do I install a wireless networking hardware?

My friend gave me an Hp laptop and my dad installed windows xp on it and then he decided to go with windows vista. After he installed vista, i tried to connect to the internet with wireless, but no networks would show up. then i put diagnose problem and it says "this computer does not have a wireless network adapter installed and configured---install wireless networking hardware and verify that the correct drivers are loaded"...i dont know what to do..
Please help me..