Currently Being ModeratedNov 19, 2012 8:54 AM (in response to Jbowlick)
A good place to start is:
Restart the computer off the OS 10.5 install DVD, run Disk Utility ad Repair your normal hard drive. Then Repair Disk Permissions.
If you had the original, came with the computer when shipped new from Apple, install disks, you could run the Apple Hardware Test to see if there was a hardware problem.