cannot boot up iMac with 10.6.8 Snow leopard

iMac 10.6.8 cannot get to boot up. Had been getting 'startup disks full' but didn't know what to do about it. Now, it boots up to the blue screen, apple, with the spinning disk, and the horizontal tube marker. That tube gets about 1/4 inch full, and then erases it - and starts it again, again geting about 1/4 inch full, before it shuts completely down and goes black screen. All that takes about 3 minutes.

I tried to hold the shift key down, but doesn't change anything. I have the original Snow Leopard disc I upgraded from Leopard.

Any help would be greatly appreciated! I am sending this from my older MacBook - also running 10.6.8.