You could also have fake anti-virus installed (Malware) which is terrible. Be sure to have installed a PROPER anti-virus like AVG or Kaspersky before downloading/searching the web. (AVG free is good even if it is free.) Just to make sure: I downloaded Mythruna today without a problem.
I do have an actual antivirus. BullGaurd. It was listed as an actual antivirus by windows 7.