Many destructive viruses damage the Master Boot Record (MBR). A damaged Master Boot Record means that the operating systems cannot be loaded and  make it impossible to start the computer from the hard disk.
If you wonder how to backup and restore the MBR, an easy solution could be found in a freeware tool called MBRfix.
MBRFix is a command line tool for Windows and has many advancedd commands that allow to repair, backup, restore and manage MBR.

From sysint.no website:
Perform several Master Boot Record (MBR) tasks, like backing up, restoring, fixing the boot code in the MBR, etc.

Requirements:

Windows NT, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008, Windows 7 and Windows PE. Both 32-bit and 64-bit x64 editions are supported.

Commands:

    MbrFix /drive <num> driveinfo              Display drive information
    MbrFix /drive <num> drivesize              Returns drive size in MB as return value
    MbrFix /drive <num> listpartitions         Display partition information
    MbrFix /drive <num> savembr <file>         Save MBR and partitions to file
    MbrFix /drive <num> restorembr <file>      Restore MBR and partitions from file
    MbrFix /drive <num> fixmbr {/vista|/win7}  Update MBR code to W2K/XP/2003, Vista or Win7
    MbrFix /drive <num> clean                  Delete all partitions on the selected disk
    MbrFix /drive <num> readsignature {/byte}  Read disk signature from MBR
    MbrFix /drive <num> writesignature <hex>   Write disk signature to MBR
    MbrFix /drive <num> generatesignature      Generate disk signature in MBR
    MbrFix /drive <num> readstate              Read state from byte 0x1b2 in MBR
    MbrFix /drive <num> writestate <state>     Write state to byte 0x1b2 in MBR
    MbrFix /drive <num> readdrive <startsector> <sectorcount> <file>
                                               Save sectors from drive to file
    MbrFix /drive <num> /partition <part> fixbootsector <os>     
                                               Update Boot code in boot sector
    MbrFix /drive <num> /partition <part> getpartitiontype
                                               Get partition type
    MbrFix /drive <num> /partition <part> setpartitiontype <typenum>     
                                               Set partition type
    MbrFix /drive <num> /partition <part> setactivepartition
                                               Set active partition
    MbrFix /drive <num> getactivepartition     Get active partition
    MbrFix volumeinformation driveletter       Get volume information for partition
    MbrFix flush {driveletter(s)}              Flush files to disk for partition
    MbrFix listpartitiontypes                  List partition types

Link : MbrFix