I just got a PowerBook G4 from a friend and it doesn't startup. When you turn the power on, it sounds the little Apple diddly and loads a white screen with a folder image in the center. The image in the center of the folder changes from the Finder icon to a question mark repeatedly. It will do this forever and not startup, any ideas?

If you still have your OS X installation disk, you can most likely repair your system. Boot off it by inserting it into the disk drive, and holding down the 'C' key while the computer starts up. After you enter the installation program, from the 'Utilities' menu, choose 'Disk Utility'. You can then run disk repairs on your startup volume which will (hopefully) fix the problem.

If it doesn't, I'd say your next best option is to reinstall the operating system. If you haven't got any important data on it, do a complete wipe of the hard drive. If there's stuff you want to save, make sure you choose "Archive+Install" as the installation type. All your documents and settings will be preserved, while reinstalling the critical system components.