se você estiver usando o grub, mas não estiver detectando o Windows, tente:
sudo update-grub
o resultado será como:
Found linux image: /boot/vmlinuz-3.19.0-21-generic
Found initrd image: /boot/initrd.img-3.19.0-21-generic
Found linux image: /boot/vmlinuz-3.19.0-18-generic
Found initrd image: /boot/initrd.img-3.19.0-18-generic
Found linux image: /boot/vmlinuz-3.19.0-16-generic
Found initrd image: /boot/initrd.img-3.19.0-16-generic
Found linux image: /boot/vmlinuz-3.16.0-38-generic
Found initrd image: /boot/initrd.img-3.16.0-38-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Windows 8 (loader) on /dev/sda1
done
se você encontrar o carregador nele, então está funcionando