Installs from downloads are not working?

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http://askubuntu.com – I just installed Ubuntu (though I'm about to reinstall to make it my only operating system) and there are two things I want to install: Avast! and Chrome. When I download the files I download the .deb versions and the Software Store (or whatever it is called) will start the install correctly. However EVERY SINGLE INSTALL returns the error that the package is corrupt. However if I choose something from the software store it installs without issues. Why are my installs failing??? (HowTos)