My iMac does not seem to be able to boot from a CD. I have tried to start up from both a TechTool Pro disk and a Tiger OS disk. I know that there are multiple things that could be wrong with the TechTool disk (software needs to be updated, is the disk compatable, etc.). However, there is no excuse as to why the OS disk won't boot. When i restart with the disk, the computer does nothing for a while, then the grey apple sign with the "spinny" thing appears for a while, and then the grey apple goes away and a "non-smoking" sign appears. Whats going on here?