Bitlocker

On a computer, protecting your user account's data with a login password only protects against unauthorized login but not against anyone who has physical access to the computer. This person can simply insert a bootable device into the computer's USB port, boot from it, and have full access to the data on the computer. This means if your leave your computer unattended for a few minutes, there might be enough time for this person to steal your data without your even knowing because he does not need to walk away with your computer.

However, when the harddrive on your computer is encrypted, any unauthorized person who has physical access to your computer cannot access the data on it because the entire drive is encrypted. With that said, you can just encrypt your user's profile to get the same protection but this only affects your profile. If you per chance store sensitive data outside your user profile, then there is no protection for it when the unauthorized person has physical access to the computer.

On a Windows 10 machine, whole drive encryption can easily be done using the built-in BitLocker. Setting it up is very simple and straight forward. However, it does require that the Windows 10 is Professional or Enterprise version. If your Windows 10 is the Enterprise version, the computer is most likely maintained by your company's IT department. As such, it may already be encrypted with either Bitlocker or some other method. If not, talk to your IT department if you are concerned about data breaches.

The steps to use Bitlocker to encrypt the drive can be found on the Internet including myriad videos on YouTube. As with all things with computer security, back up your computer's data before trying anything. There is a chance that ironically, you lock yourself out of your own data so the backup is the insurance. In general, you need to back up valuable data regardless.

Windows Setup: Installing using the MBR or GPT partition style

https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/windows-setup-installing-using-the-mbr-or-gpt-partition-style

In general, when you install Windows 10, you should install it under UEFI mode to get additional features. Other than making the boot up faster, it also enables multi boot with other operating systems such as Linux. Each OS needs its own partition. With GPT disk, you can get more than 4 primary partitions. Moreover, given that ChromeOS is all the rage these days, an existing UEFI Windows is needed if you want to dual boot with Neverware's wonderful CloudReady ChromeOS that you can install on any old PC for free. Granted, if the PC is too old, its BIOS cannot run UEFI anyway. In short, if your old computer's BIOS support UEFI and can run Windows Vista or later, you can dual boot it with CloudReady ChromeOS.