Boot into your Ubuntu LiveCD on the offending machine. Once Ubuntu starts up, go to System -> Administration -> Software SourcesUniversal and enable (by checking it off) the repository.Now, open a terminal session (Applications -> Accessories -> Terminal) and type the following:sudo apt-get install ms-sysms-sys is a program used to write Microsoft compatible boot records.
Now you’ll need to figure out what partition is the one hosting your Windows operating system. Back in the command line, type:sudo fdisk -lThat will list the available partitions. You’re looking for a partition that says something like/dev/sda1 1 9327 74919096 83 NTFSThe two important bits are the ‘/dev/sda1‘ which is the partition itself and the ‘NTFS‘ which tells us it’s a Windows formatted partition.We want to fix the MBR, on /dev/sda1. To do so, type:sudo ms-sys –mbr /dev/sda1You’ll want to change the ’sda1′ bit if your results from ‘fdisk -l‘ are different.Once you do that, reboot the machine, removing the LiveCD from the drive and Windows should come back to you.Sure, you could do this by inserting the correct Windows CD and booting into repair mode from it - but I find the Ubuntu way a bit faster and I’m more likely to have an Ubuntu LiveCD on me than a Windows CD.