power mac g5 will not boot up

Hi,
I reasently got myself a power mac g5 and tried to update the software to 10.5 but it froze at a screen message saying 1 hour and 25 minutes to install and stayed there for hours. I instructed the mac to cancel update and excepted the instruction to restart. Now it wont startup sticks at the apple logo and spining disc.
Can someone help as i am unsure which way to go to fix this problem.

Cheers.

Can you boot from the Leopard install DVD by holding down C at startup as the install disc must have been in the optical drive? Take it the install discx is a retail install DVD and not a model specific one which comes with a new computer?