Most likely issues
Issues that can cause the issue are not limited to but can cause this are:
Dead PRAM battery....that will be fun for you to replace.
Dead or dying Hard drive..... Just as fun as the battery.
Or your software just needs reloaded, try booting from CD and repair disk then permissions. If it won't do that refer to the first 2.
Good Luck. I have had a few, mac fixit has a great take apart and it's not too tough. Just remember to get a new tube of thermal paste and take it all the way apart so you can clean the fan at the top because it will be caked with dust bad if it was ever used.