I just replaced my hard drive and I can't boot from the CD drive!

Hi! Does it let you go into option mode, which lets you select from available bootable devices? Try booting up while holding down the option key. Do you have any other bootable media to try, and have you been able to boot from this CD before?

Also, it's not uncommon for a laptop to have an "open firmware password", meaning that someone decided to "lock" the computer as a security mechanism. A sign of this is that it won't boot from another device. It may be that the previous owner did this, but since you have the password to log into the OS, you've never had to boot from CD media before.

If this is the case, the solution is to temporarily change the amount of memory in the machine by either removing the extended memory, or adding memory if there is no extended memory, and then powering it on and IMMEDIATELY resetting the PRAM three times. It is critical you reset the PRAM the first time the machine powers up after the memory change, or the machine will have a chance to register the change and the magic voodoo that kills the password lock will not happen. Reset the PRAM by holding down option-apple-p-r and then powering on with those 4 keys held down. The screen will be black. After the initial power-on chime, wait until you have heard it chime 3 more times, then release, and then hopefully it will boot from the CD, let you do stuff from option mode, etc.

Good luck!