I had to re-install the Debian bootloader after installing Windows 7 Ultimate and I did this by loading up my OpenSuse x64 installation DVD and selecting the rescue option which gave me a login prompt. I logged in as root and after following this post: Gentoo chroot guide, and setting up a chroot to my Debian installation I could then type this command in the root prompt.
root@locahost~# sudo grub-install /dev/sda |
That re-installed the proper grub bootloader to the hard disk MBR. The Debian installation DVD would have been useful for this, but I did not have that to hand. If the installed operating system is 64 Bit then the live CD has to be also. I have used this method a few times to re-install the bootloader to the MBR after it has been overwritten, this works perfectly. Windows does overwrite the GRUB bootloader when it is installed, it does not care about other operating systems at all. At least, Ubuntu or Debian discs offer a way to fix this very easily. Should not have to worry about this though, but Windows does not use GRUB.
At least Linux makes a very good rescue disc when a command prompt is available on the boot CD.