Veracrypt

The Windows system partition contains everything that you see when you open your C: drive in Windows Explorer: Windows system files, documents, user accounts, etc.

When you encrypt the system partition, all those files get encrypted with whichever algorithm you choose.

Anyone who gets a hold of your computer will be completely unable to access anything on the C: drive because it's all been encrypted.

HOWEVER, they would be able to access it if your computer gets stolen while it's powered on or in sleep mode (if it's a laptop)

So if you're really paranoid, you should always shut down your system when you're in a situation where it could possibly get stolen.

No. That's the whole point. If you only have one disk (or volume), both the operating system and your documents are on it. Therefore, encrypting it means no one can use your computer unless he has the Vera Crypt password.

It encrypts the entire C:-Drive but not the bootloader. So if theoretically someone gets access to your physical hard-drive he could only know which OS you are using from the bootloader but he couldn't access any files on the system-partition..