Clicking noises can be an indication of a failing hard drive. It's the reader head going back and forth. It can also be a very fragmented drive and the system data may be all over the drive and it's having a hard time finding it. How full is the drive? Does it boot from the original system installation DVD? Have you backed up your data to another internal or external drive, if not do so ASAP. Run Disk Utilities on the drive from the installation DVD.