why can't I boot from the installation disc of 10.4

I'm now running 10.3.9 on my eMac. And lately I want to upgrade it to 10.4 I've tried different versions of installation discs but none of 'em can lead the boot. I've tried either clicking on the install icon in the disc and reboot or set the DVD-ROM as boot device.
I also tried press "C" when the system boots but it still doesn't work. The system doesn't seem to be able to recongnize the disc.
What could be the problem?
What can I do?

By the way,the installation disc is made from ISO image or DMG image from internet free source. And I burt 'em with Superdrive on my eMac(I don't have a DVD burner on my PC).Could this be the problem ?

buy the upgrade from apple. if that doesn't work, post back :]

I'm quite sure the ISO image is fine,'cause it's been run by many others. The question could be focused on the way I burned'em.
I burned 'em on Mac OS operating system, which could cause the file leading the boot get erased, I suppose.
Any suggestions?

this forum does not support 'pirated' or 'freely downloaded' software, so the only suggestion i may offer is one of legality which is to buy the upgrade from apple.

it works if u mount it with daemon and vmware but when u burn it, it seems to cause problems. I don't know why.