i get the sign on page but it wont accept my password

bit serious i know , but ive updated my i mac to leopard ,and for the first time in months i've shut it down .
When i came to use it again i get the sign on page but it wont accept my password .(i get the shake screen)
i've tried inserting install disc on start up and restart.
pressing apple c, apple r, apple s.
can anyone help ?
please.

Go to this page and scroll down to the bit that says
Resetting the original administrator account password

Mac OS X: Changing or resetting an account password

hi louishen ,thanks for the speedy reply, ive tried that and got a grey screen with
panic(cpu0 caller ox002e54bc) unable to find driver for this platform

"powermac8,1 "


and more writing with the kernal panic box in the background.

To reboot from your Leopard disc, pop the disc in and reboot. Just hold down the C button after the chimes and not the Apple button.