I had this exact problem not but a month ago. Something about this particular iMac i guess. Nothing I did worked. I tried every trick I could find: booting from various install CDs, starting up in single user mode, dismantling the thing and putting a new hard drive in it. Nothing at all worked. What I finally did was I took the original hard drive out of it and hooked it to another working mac i have with a USB IDE drive adapter and ran a disk repair on it from there. After that it booted, no problem. Go figure.