Those who are unfamiliar, the MBR or master boot record is a 512-byte boot sector that resides in the first sector of a hard disk and which is responsible for bootstrapping your operating system into memory and holding a disk's primary partition table. The MBR is essential for booting into the operating system, Windows in this case.
If the hard disk contains a single operating system, there is no real need to backup the MBR because the MBR can be easily repaired from Windows Recovery Console using the fixmbr command for Windows XP or by running the bootsect command for Windows 7.
Thing are different if you have multiple operating system or Windows versions installed on the disk. In this case, the MBR contains record of all the OS present on the disk and is capable of booting into any one of it, according to which the user selects in the boot menu. Repairing such an MBR through the recovery console will wipe it clean and you will be unable to boot into the other operating systems.
So the MBR on a multiple-boot system need to be backed before carrying out any system changes, such as reinstalling the primary OS, if you wish to keep the other OS intact.
MBRWizard is a command line tool that allows you to save, restore and repair the MBR among other things like creating new partition, aligning partition, deleting partition and so on.
Download MBRWizard CLI tool from this page (click on the CLI Freeware tab). Open Windows Command Prompt and navigate to the location where you have saved MBRWizard. The easiest way to do this is to drag the executable file and drop it into the Command Prompt window to automatically populate the directory path. Alternatively, you can copy the MBRWizard executable to Windows System32 directory. This will allow you to run MBRWizard from any location.
Either way, after you have browsed to the location of MBRWizard, enter this command into Command Prompt.
MBRWiz /save=MBR /filename=D:\backupMBR
This will backup the MBR to D:\backupMBR. Of course, you are free to choose your own location.
To restore a saved MBR issue this command:
MBRWiz /restore=MBR /filename=D:\backupMBR
Related: PLoP Boot Manager