imac,extension ON won't boot,extension OFF it work,help

[krisoto]

i recently try to do a clean installation again but whenever i start to reboot it it freeze after the happy face and it stop on 2 icon at the bottom( question mark and the sound) i change the extension to mac os 9.1 base, i ran the first aid disk 8.6 and i i boot from cd to format the hard drive but it's still going back to the same problem. pls. help