Ferden you never mentioned what AV was preinstalled on your netbook when you purchased it and how it was removed, more than likely it was McAfee or norton and you should run the correct removal tool for it from here http://singularlabs.com/uninstallers/security-software/ please read the uninstall information before running removal tool, once done and all other security softwares have been removed then a fresh install of avast should work for you which you can then beef up with Malwarebytes.