De acordo com os documentos de ajuda do Ubuntu :
Installing Windows After Ubuntu
Normally when Windows is installed after Ubuntu the "Master Boot Record", MBR, will be overwritten. You can bootup off a LiveCD and repair the MBR. However, there are 2 different approaches:
Recovering GRUB after reinstalling Windows
Please refer to the Reinstalling GRUB2 guide.
Master Boot Record backup and re-replacement
Back-up the existing MBR, install Windows, replace your backup overwriting the Windows boot code:
Create an NTFS partition for windows (using fdisk, GPartEd or whatever tool you are familiar with)
Backup the MBR e.g. dd if=/dev/sda of=/mbr.bin bs=446 count=1
Install windows
Boot into a LiveCD
Mount your root partition in the LiveCD
Restore the MBR e.g. dd if=/media/sda/mbr.bin of=/dev/sda bs=446 count=1
Restart and Ubuntu will boot
Setup grub to boot windows
Veja também a pergunta do AskUbuntu e outra AskUbuntu Question e este artigo do blog .