Q: imac g4 powerpc won't boot Apple logo and busy icon no install disks 10.3.9

If you don't have an installer disc available you can try effecting repairs using fsk in Single User Mode - http://docs.info.apple.com/article.html?artnum=106214.

Post by japamac about using fsk - http://discussions.apple.com/thread.jspa?threadID=1649143

If that doesn't work then there really isn't anything you can do without installer discs.  You could try contacting  Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - http://support.apple.com/kb/HE57). Provide the serial number and specifications of the Mac.  They may be able to provide a replacement set of discs for a fee.  These discs will be for whatever OS was installed on the computer originally, so if you have an upgraded version you will need to obtain the discs some other way.