Most Distros, when installed, would modify the MBR to boot their associated Lilo or Grub bootloader.
I have a query though... If you have an .iso on /dev/hda this will be an image file which you would have to burn to a CD or DVD before it would be of any use to you. You'd then boot the CD/DVD to carry out the install. Is this what you meant or are you trying to go directly from the .iso to install?
I don't think you can do the latter.