I am trying to download softwares from ubuntu software center but every time i click on "USE THIS SOURCE" i end up getting "Failed to download repository information" error! I don't understand what to do I even tried changing and adding Repository. Even tried choosing a different server. Even the "Choose best server" option doesn't choose anything. I really need help. I am new to Ubuntu and i would really appreciate some help.

after trying lsb_release -a; i got

pooja@Invincible:~$ lsb_release -a; 
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 12.04 LTS
Release:    12.04
Codename:   precise

After trying "sudo apt-get update && sudo apt-get upgrade" i got this-

W: Failed to fetch http://mirror.cse.iitk.ac.in/ubuntu/dists/precise-proposed/universe/i18n/Translation-en Something wicked happened resolving 'mirror.cse.iitk.ac.in:http' (-5 - No address associated with hostname)

E: Some index files failed to download. They have been ignored, or old ones used instead.