Click on the first icon in the Dock (the list of icons at the bottom of your screen) which is the “face” the "Finder", then click on Go at the top of the screen, you will see a drop down menu, go to Utilities. When that opens double click on the application called Disk Utility. When Disk Utility opens you will see your hard drive (usually Macintosh HD) on the left. Highlight your hard drive and click on Verify Disk on the far right. If that is OK in green type, then click on Repair Disk Permissions on the left.
Note: If Verify Disk is not OK try to Repair. If Repair is successful then Repair Disk Permissions if it is not successful let me know.
Then something is wrong. Do you have your Leopard install disk? We can try another way if not.