Mac OS X discs for PowerPC-based Macs don't work with Intel-based Macs
Products Affected
Mac mini (Mid 2007), Mac mini (Late 2006), Mac mini (Early 2006), iMac (20-inch, Mid 2007), iMac (24-inch Mid 2007), iMac (Mid 2006 17-inch), iMac (Early 2006 20-inch), iMac (Early 2006 17-inch), iMac (24-inch, Late 2006), Mac OS X 10.4, iMac (20-inch Late 2006), iMac (17-inch, Late 2006 CD), iMac (17-inch, Late 2006), iMac (24-inch, Early 2008), iMac (20-inch, Early 2008)
Mac OS X installation discs that came with a PowerPC processor-based Mac won't work with an Intel processor-based Mac, and vice-versa, even if both Macs came with the same version of Mac OS X. You can't start up an Intel-based Mac from a CD or DVD that came with a PowerPC-based Mac, nor a PowerPC-based Mac from an Intel-based Mac's discs, either by holding the C key at startup or by using Startup Disk preferences.
To start your computer from a disc, use the disc that came with the computer.