Well, it all has to do with the types of ads and sites that it tries to expose you to. While a lot of the advertised content would be legitimate, since there’s not much control over the sources of what gets advertised, it’s still possible that some of the sites you may eventually get sent to by the hijacker may not be particularly safe. We probably don’t need to tell you just how damaging and problematic malware threats such as Trojan Horses, Spyware, or Ransomware could be, but this is exactly what you may end up getting on your computer if you allow MainWindow to continue spamming you with its obstructive and aggressive pop-ups, banners, page-redirects, and clickbait links. Fortunately, removing this hijacker is not too difficult of a task and you should be able to manage this on your own, provided that you follow the uninstallation instructions prepared at the bottom of this post.