Many free pirate movie websites spread viruses, malware or adware. Pirate streaming sites tend to make their money from advertisements, and since piracy is often illegal itself, they are unscrupulous about which adverts they show. Some of these sites show advertisements that spread viruses or trick users into downloading malware.
The safest option when finding a free movie website is to always prefer websites that are authorized to stream movies. Companies such as Crackle or Hulu are recommended by DigitalTrends. There are also safe websites that specialize in legal streaming of free movies and documentaries, such as SnagFilms. If in doubt, it is best to check a streaming website using an antivirus website checking service, such as Symantec SafeWeb.Norton.com.