How to make your computer unvulnerable to viruses?

Viruses represent a threat to computers. Such famous malware as WannaCry and PETYA infected millions of PCs, causing irreparable harm. They are not so spread anymore, but there are thousands of dangerous viruses out there, which are exploting the 0day vulnerabilities. Some day, such virus can get out and instantly spread across the world with any antivirus program being unable to overcome it. So, what can be done?
The first and most obvious answer is not to use internet at all, but this is a rather foolish decision, as all the sense of using a compter is having an access to internet.
When analysing the viruses (WannaCry and PETYA in particular), I noted a very interesting peculiarity. They both use path to get to the system files. It means that their attack is aimed at drive C:/ - the part of your hard drive where system is installed. Therefore, all the system files are located there. So why not just replace the system to part D:/ and live happily? That's exactly what I suggest. You see, you can't just istall windows on C:/. There's not such function by default, but you can use special programs like BOOTICE toperform the task. You can actually find manuals in google.
So, if it is so simple to prevent virus atacks, why so few people know about it? And why microsoft doesn't allow to install windows on D:/ by default? The thing is, if everyone knows about it and uses this measure to protest from viruses, hackers will make viruses that infect irrespectively of where the system files are.
So, should we install system on other particion of the hard drive? I think yes. As long as it is enough difficult to do, very few people will do it and hackers won't pay any attention to them as to the minority.