Fix a Windows MBR with an Ubuntu live-CD
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1.  Boot into your Ubuntu LiveCD on the offending machine. 

2.  Go to System > Administration > Software Sources and enable (check it) the Universal repository.

In Terminal type:

3.  sudo apt-get install ms-sys             # ms-sys is a program used to write Microsoft compatible boot records

4.  sudo fdisk -l                           # figure out what partition is the one hosting your Windows operating system
                                            # The is partition similar to  /dev/sda1 1 9327 74919096 83 NTFS
                                            # 'NTFS' which tells us it's a Windows formatted partition.                 

5.  sudo ms-sys -m /dev/sda                 # Note: the Windows install is usually found on on sdb or hda.

6.  Exit from Ubuntu and reboot the computer -- hopefully back into Windows.