It's perhaps easier to take a portable Linux distro (tutorials can be found here) and follow these steps from Arsgeek.com
Boot into your Ubuntu LiveCD on the
offending machine. Once Ubuntu starts
up, go to System -> Administration ->
Software Sources and enable (by
checking it off) the Universal
repository.
Now, open a terminal session
(Applications -> Accessories ->
Terminal) and type the following:
sudo apt-get install ms-sys
ms-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 -l
That will list the available
partitions. You’re looking for a
partition that says something like
/dev/sda1 1 9327 74919096 83 NTFS
The two important bits are the
‘/dev/sda1‘ which is the partition
itself and the ‘NTFS‘ which tells us
it’s a Windows formatted partition.
So your Windows partition exists on
your drive sda and it’s partition 1.
The MBR for drive sda (assuming you
boot into windows using it’s native
boot loader) is what you want to
repair. We want to fix the MBR on
/dev/sda. To do so, type:
sudo ms-sys -m /dev/sda
You’ll want to change the ’sda’ bit if
your results from ‘fdisk -l‘ are
different. If for instance your
windows install is on sdb or hda. Once
you do that, reboot the machine,
removing the LiveCD from the drive and
Windows should come back to you.