Hello, welcome to JustAnswer, my name is Toby and will happy to assist you.
So you have been unsuccessful booting from the DVD? To be sure, are you powering on your Mac and pressing and holding the C key until you see the Apple logo?
What version of OS X is installed, Snow Leopard (10.6), Lion (10.7), or Mountain Lion (10.8)?
If you would like to troubleshoot this issue, please respond and the system will alert me.
10.6 is the version that i have.
To boot from the DVD, are you powering on your Mac and pressing and holding the C key until you see the Apple logo?
and yes i have hold the c key then nothing else happened but the logo appears for 10- 15 seconds and then the sign "forbidden"
I'm not sure if you have reset the PRAM, but let's try doing that to see if that will help you boot from the DVD.
Turn off your computer.
yes i did ... did not help
From what it sounds like, you may an issue with the system/logic board. You will need to take this to the Apple Store or find a Apple technician in your area.
If you have any other questions, please let me know.
ok thanks i'll do that