See these SSDs? In a typical system you only have one of them: SSD A (can also be a hard disk). It has your Windows and your files: Your personal stuff like financial data, health information, your nude pics, your passwords and so much more!
With it you do your taxes, receive emails, surf the web, and so much more.
You connect this PC with all your personal data to websites that might be infected.
Are you worried? Probably not. (If you were, you would not be using Windows.)

Here’s the thing: Using this PC to surf the web is like sticking your hand into the behind of an infected person while hoping the rest of your body will not be affected.

Let’s consider a different setup:

You have 3 SSDs in your computer. They are connected to a switch. Now you can select which one to boot from!
Why is this good?

SATA switcher. Found on Amazon.

You could use SSD A, the one with the spyware called Windows, for things that only run on Windows, like certain games. For nothing else. Because it is not safe.

You could install a Linux on SSD B. This could be your main operating system. Where all your important stuff is. Safer than Windows. Also: you would not use this system to widely surf the web, only certain sites, only with tight security (NoScript, for the insiders among you).

And now SSD C: You would install a Linux on it (can be the same one as on SSD B). On this system you would have NO personal data and you would NOT visit websites where you have to login with personal credentials.
This SSD is only for surfing the web with relaxed security so all the websites work. Who cares if it gets infected? Since the other SSDs are switched off, they will not be in danger. Your data is safe.

That is why you need a SATA switcher. That’s why your main PC needs to be a desktop, where you can install such a switcher.

You can buy one for around 20 Euros or build one. I made this primitive one with relays.

(If you need a software solution for this problem that also works on laptops look at Qubes OS.)

(SATA is the interface on SSDs that connects them to your system.)