There are many ways but a favorite way is to boot any Linux LiveCD and to use the syslinux package like so:
Just change the "sdx" to your sd for example /dev/sda or whatever the drive is that is supposed to boot Windows.
sudo dd if=/usr/lib/syslinux/mbr/mbr.bin of=/dev/sdx
0+1 records in
0+1 records out
440 bytes copied, 0.0197808 s, 22.2 kB/s