ignore it for blender.org. if you get bleender from blender.org its safe and i’ve never heard of one from graphicall containing viruses. get a good anti virus program and scan every program before you install or even open it. cnet downloads is also another safe place to get software from, they scan it for viruses before they put it up for download.
avast has the ability to scan files, even zipped ones, before they are opened. and you can get that from cnet. microsodt will throw up that warning about any software that hasn’t went thru its certification, and they aren’t going to waste the time, effort, and money on a software that gets updated as often as blender. the authentican mesage dosn’t mean its bad software, just that ms dosn’t know, and thats why you check everything with a good antivirus before you install it.