Unless someone else has a better idea, as a test I would try taking avast! temporarily out if the equation to see if the problem persists.
If you can download another free av (eg AVG or Antivir trial version) while avast is still installed do so, take the computer offline, uninstall avast!, reboot, and install the alternate av.
If this can't be done try downloading to CD from a computer that is not experiencing this problem, then unistall/install.