DVD reading Problem

Guys give me some idea
Mine is iMac G4 , 10.4.9 and my computer does not read any DVDs not even operating system DVD
if i insert blank DVD then it shows as blank dvd but not movies or anything else not even the dvd which i Write in my computer
i want to boot and reinstall Mac osx10.4
What do i do?
Please help

Tried rebooting with the Install DVD in the slot/tray and holding the "C" key when you hear the startup chime?

If it works, try repairing Permissions/Disc using Disc Utility in the Utilities Folder on the install disc.

I tried that but it starts up normally
it doesnt show cd icon on desktop when i insert

Open System Preferences and look in the System section for Startup Disc and see if the install disc shows up there when it's in the drive. If it does show, select it as a Startup Disc and restart.

Also look in Disc Utilities to see if the Disc is showing up in the left pane.

Another thing to check is Finder -> Preferences and make sure that in the General section there's a tick in the CDs, DVDs, and iPods of "Show These Items On The Desktop:"

i tried that one as well
On System Pref> System> start up disk > it only shows "Mac OsX 10.4.9 on 3" and "Network start up"
On disk utilities It doesnt show Disk on left panel
on Finder> Pref>it is checked on cd,dvd,ipod