Normally a clicking sound coming from your hard disks is a Bad sign. It could be an indication of terrible hardware happenings, thus my instant advice is backup your hard drive once possible actually, even if you don't suspect a problem with your hard disks, it's wise to make regular backups so you can survive a hard drive crash with only negligible problem.

On Windows XP, you can test your hard disk for errors by running the CHKDSK utility. Restart your pc and hold down the Ctrl key as it's booting up. When you see the selection to start your system in Safe Mode, select it and continue. Click the Start button, then click Run, then type CHKDSK C: /F /R then press Enter. The /R option tells CHKDSK to scan for bad disk sectors, and the /F option means to fix any errors found.