Please go to the Apple logo at the upper left of your computer screen to About This Mac and give me this information: The exact system Version (listed right under Mac OS X) Processor Memory
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.
Put the Snow Leopard install disk in and restart holding down the letter C key until you see the Apple logo. Choose your language then you get to the install screen DO NOT INSTALL, you will see at the top of the screen a Utilities menu drop down. Go the Disk Utility, Highlight your hard drive (usually Macintosh HD) and click on Verify Disk on the right. If that is OK and only if that is OK click on Repair Disk Permissions. 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