My late 2006 iMac seems to have died. When i press the on button it starts up normally with the sound and the apple symbol on the screen with the wheel starting to spin. then the computer just dies. i have tried to insert system disc but there is no significant change. i can hear it reading the disc but then it just dies all the same. also tried booting with an external hard drive but withoiut result. any ideas? what could be the problem? i recon there is a hardware failure but where?