I can't install any package using apt-get install or using the ubuntu softare center

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http://askubuntu.com – I installed recently in my personal computer unbuntu 12.04... When a try to install some software (Dropbox or chromium per example) from the ubuntu software center or when i try to install some package using the terminal (sudo apt-get intall guake ... per example), i find an issue... there is some problem with the packages list. What i can do? I tried run sudo apt-get update, but the issue persist. thanks. (HowTos)