How do I connect my XP laptop to a wireless network?

Right, lets see if I can explain this. I have a Vista laptop connected to a Buffalo wireless router which all works great, but how do I connect my XP laptop to the network, so it can go online via the router and transfer files to the Vista laptop? I hope that makes sense - can someone explain this (in simple terms!) or point me towards a guide of some sort?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).