I try downloading the launcher for Windows, click "Run", and get this "Windows cannot access the specified device, path, or file. You may not have the appropriate permissions to access the item" message, and nothing I do seems to change this. Tried downloading AIR, no luck. I also get a notification on my computer saying it has been blocked "Due to malware".

You need to tell your computer that even though it thinks it's malware, to run it anyway. It thinks that because it's a new program that it isn't aware of and can't find any reference to in it's databases. So in the interest of being super safe, it has blocked it for you.