"Unable to locate package" errors for all software

My apt-get doesn't install any programs. I try to install programs with apt-get install but always it shows same error: unable to locate package.
For example I try to install the xbmc media center; here's what happens:

$ sudo apt-get install xbmc    

Reading package lists... Done    
Building dependency tree           
Reading state information... Done    
E: Unable to locate package xbmc

I have updated my package list with sudo apt-get update.