Before I dispose of my imac caught in password loop

Before I dipose of my imac I'm trying to uninstall 10.4 to erase my data (I have the install disk in the computer) but I'm caught in password loop. I am hoping that when I restart while pressing the "C" key that I will be offered an option to uninstall but instead I am asking to enter my password. When I enter it the computer restarts and takes me straight back to the password prompt again. I hope that someone can help!