How to backup MBR on Windows?

You can use PowerShell, which comes with Windows 7 and newer. This script uses P/Invoke to access the native CreateFile function, opens the first disk for direct reading, and saves the first 512 bytes of the disk to a file called mbr in the current directory.

Add-Type @"
using System;
using System.Runtime.InteropServices;
public class PInvoke {
    [DllImport("kernel32.dll")] public static extern IntPtr CreateFile(string lpFileName, int dwDesiredAccess, int dwShareMode, IntPtr lpSecurityAttributes, int dwCreationDisposition, int dwFlagsAndAttributes, IntPtr hTemplateFile);
}
"@
$handle = [PInvoke]::CreateFile('\\.\PhysicalDrive0', 0x80000000, 6, 0, 3, 0, 0)
$fs = New-Object System.IO.FileStream ($handle, 'Read', $true)
$b = [array]::CreateInstance([byte], 512)
$fs.Read($b, 0, 512) | Out-Null
Set-Content .\mbr $b -Encoding Byte
$fs.Dispose()

To use it, save it to a PS1 file like backupmbr.ps1. After following the instructions from the Enabling Scripts section of the PowerShell tag wiki, you can invoke it from an administrative command prompt:

powershell -file .\backupmbr.ps1

If you have multiple drives, you can look in the Disk Management snap-in to find the number of the one whose MBR you would like to back up, then update the number in \\.\PhysicalDrive0 to match.