Are you trying to log in to open networks (yes I know there are lots of them about) or are you trying to use the internet connection of a deliberately shared access point where you are charged a fee for internet access.
At a guess your machine is hooking onto an open network which is looking for authentication. The most common way of preventing access to these networks is by allowing a particular list of machines (using their MAC address) access to the network.
If your not on the list you ain't getting in...