That's just an issue with Microsofts' Windows. Its just a warning saying that something with your download is not known to the database of pre-trusted download sites. Its not really something to be concerned with. It's an attempt by Microsoft to let users become aware of when they are downloading files from sites not "approved" via MS Windows.
Listing every single approved site would be impossible.
Certificates lapse, expire. New sites spring forth. As long as you are wanting to download a program from a known site to yourself. And you trust the site, or are running very good Anti-Malware software then accept the certifcate, trust it, and get your file.
If you let a warning like this, which will happen more and more often as MS Windows grows more outdate, you will be crippling yourself to what you can do on the Internet.