I'd recommend following through with system restore for your issue. Click me! (http://windows.microsoft.com/en-us/windows7/products/features/system-restore)
Also, like Thunderboltsiren asked, have you recently downloaded anything? Has anything suspicious been occurring? Try scanning your PC with this:
MalwareBytes (http://www.malwarebytes.org/)
It's very strange that your PC would suddenly deny your download.
Also, try downloading FH's setup from another browser. (Google Chrome, Firefox, etc.)
Your download was possibly corrupted, but that's not common.
Good luck!