This is the MacBook forum - you'll want the PowerBook forum.
At any rate, if you have the Apple hardware test disc, try putting that in the drive on boot and holding the C key down until you see the Apple logo and then run the test itself when you can. That may point out the source of the problem (corrupt hard drive, bad memory, and so on). Alternately, booting from the OS X disc that came with the Mac may be a way of seeing if it's the hard drive. If you can boot on a basic level from the OS X disc, the hard drive or the OS install is at fault.