To fix your /etc/network/interfaces file, go to a terminal
(Application > Accessories > Terminal) and enter the following command:
sudo gedit /etc/network/interfaces
This will launch a text editor with the interfaces file opened in it.
In this file, put a comment (#) in front of EVERY line of the file
EXCEPT for these two lines:
iface lo inet loopback
Click SAVE on the text editor and reboot your computer.
It should now boot faster and connect to networks more easily.
Unless you are dealing with static IP addresses (which is pretty rare for the
general end-user) you should NOT establish a network connection by going to
System > Administration > Network.
Instead, you should connect to the Internet using the Network Manager icon
in the upper right corner of your screen (the one that looks like a little