Who ever installed 10.4 did not install the os9 drivers on the hard drive to have classic. If you really want to put os9 on it..you can nuke the drive and start over or use an os 10.4 disc to install the classic drivers and try to put os9 back on if you have the HD space. When you open the os9 folder find drive setup and you can try to reformat the drive. Os10 will be gone. It may not let you erase the start up drive. Thats way I take out the drive and hook it up to a PC and nuke it,then put it back in the Mac. Then you would restart holding down the c key. And with drive setup reformat it and install the Mac os.