Disclaimer, do at own risk
I intend to show short sharp instructions with more pix to illustrate to newbies how to do stuff. I know there are other ways, and I will have a sister howto on live cd. For these pix, I ran various distros in virtualbox to allow easy snapshots to be taken.
Change bios boot order to allow optical drive to boot before hard drive
boot your normal install cd or dvd....and hope its got a rescue mode. It may be called recovery.
after the rescue system has booted you normally arrive at some menu.
It is highly likely you have no mouse at this stage so use the arrow keys, the ESCape key, and my favourite TAB key...along with ENTER of course.