Try it again and take a screenshot of the error message. Let's see where it's coming from. Sounds like malware trying to stop you from getting rid of it, but it might be something else.
Also, if you're trying to upgrade to Windows 8, what operating system have you got now?
Try downloading in safe mode with network connection?
Also download malwarebytes setup file rename it and when installs rename where it wants to install the files so everything is a different name/path.
www.malwarebytes.org. Ensure you use the free and not the trial of the full version.