What are you trying to download? Also, make sure you are actually downloading from Mediafire, NOT Adfly. In the top right corner of Adfly, there is a countdown. When it reaches 0, click 'Skip ad.' Then Mediafire should popup. Click the download button in the middle of the screen and save to desktop.
If you are trying to download FH, I can assure you FH has NO viruses or malicious software.