Le Pen,
Jinkster is right - go to START -> Connect To -> Wireless Network Connection
It'll show you any wireless networks in range, and will try to connect to the best one. If the network is secure then it'll ask you for the network key, if not it will connect.
With the Wifi networks as described by Jinkster, the first time you open your browser you get redirected to the home page saying "Pay to use this network" - you enter your CC details etc and off you go. Some airports, like Dublin, allow you to buy pre-pay vouchers to use.
One thing about wifi is that the signal can vary, so being connected while sitting in the airport coffee bar doesn't mean that you may stay connected when you move to another seat in the lounge area - your laptop will tell you about the signal strength.
Have fun