Wifi doesn't work but Skype does (and it's not like the one case you already solved)
To find the hosts file, open Finder and, in Finder’s menu bar, select Go > Go to Folder. In the box, type the following location and press Return.

/private/etc/hosts


That will show you the Host file.

Double click on it and you will see:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
255.255.255.255 broadcasthost
::1             localhost 
fe80::1%lo0 localhost


Yours has to be exactly as this, nothing more or less.

If it is different, let me know so we will fix that.