how to reset the root password

I forgot completely the root password of my iMac that I dedicated to my kids for a long time, now I came to use it and found it very slow and not responsive, and I needed to login using the terminal and write some unix commands to find out the real problem, but could not remember the root password I created probably seven years ago.

as I remember that I needed to have the the CD and reboot the system with the recovery disk, but I could not get there, it always loops back to the default disk.

Any help is appreciated???

Thanks