I am using Kubuntu 12.04,currently.Recently ,I updated my system with update manager and downloaded USC from muon software center which was pre-installed in this system. I could download a lot of softwares such as:
-firefox
-XBMC media center
-VLC media player
But when I tried to get some more softwares using USC, after rebooting there began the problem . It does not begin either downloading or instaling the program. When I tried to download the same program using muon software center it showed the following:

                   Unable to obtain the package at this time  

Another application seems to be using the package system at this time. You must close all other package managers before you will be able to install or remove any packages.

I am quite sure that have closed USC . I tried to remove USC, assuming that it caused the problem using muon software center but the result was same.Now I cannot add or remove any programs into my system & I don't know what to do . Please help me as soon as possible.