If you replaced the hard drive, then that new hard drive needs to be formatted, and the system needs to be installed. A new hard drive does not come with a system already installed.
Here's what I do with old iMacs that need the hard drive replaced:
New hard drive in an external case.
Connect that to another Mac through Firewire - you can't do this external install on your old iMac, as you have no FireWire, and the USB is too, too slow - and you need a working CD drive for that to happen anyway.
Continuing on, you would install OS 9 (or OS X including 10.4 at the highest - you'll need enough RAM to make that work) on that hard drive. When the install is complete, move the hard drive back into your old iMac.
I can't think of any other methods to get a system on your iMac, when none is installed now. If your CD drive is not working, then you are "between a rock and a hard place"