Why can't my iMac's internal drive be formatted?

Okay. So ive done everything told in the books. My iMac recently just stopped working with clicking noises and random freezes. Luckily i had a backup. So i intended to reformat the internal Macintosh HD drive. I inserted my install disc and held C at the startup. I opened up Disk Utility. After clikcing erase on disk utility with Mac OS Extended (Journaled)... It begins mounting. The problem starts here when the "mounting disk" and the progress bar stops 3/4 of the way through. Ive tried waiting for an hour and no dice. Any ideas on what might be happening? Thanks.