Hi Phil,
Have you tried booting from your restore disk?
Restart your Mac while holding down the C key, pressing the power button and inserting your restore disk all at the same time. An Installer window will open, but do not proceed with any installations. Instead, from the Menu Bar, select Utilities > Disk Utility. In the Disk Utility window click First Aid, and then click Verify. If Disk Utility reports errors, click Repair. When Disk Utility is finished, from the Menu Bar, select Utilities/Startup Disk. Select MacintoshHD 10.x.x and click Restart.
If the boot disk "appears to be ok" ... how much free drive space is there? Right or control click the MacintoshHD icon on your Desktop, then click: Get Info. In the Get Info window, click the discovery triangle so it's facing down. You will see; Capacity and Available Make sure you have 10% available disk space, 15% is better. Insufficient available space, can cause performance issues, system corruption, and possible loss of data.
Some links that will provide more information on maintenance.
Maintaining Mac OS X
Leopard Cache Cleaner
Carolyn 🙂