If you have done anything to date you should list them to avoid the, I already did that scene.
Starting points:
SMC reset:
http://support.apple.com/kb/HT3964
Follow directions for your machine, battery you can remove/batttery you should not remove.
Try a pram reset:
Power up, holding the following keys:
OPTION COMMAND P R
Hold till you hear the start chime chimee 2 times, then release, reboot.
Then try a disk repair, from your original install disk, power up push in disc, select the language, and on the next screen in the menu bar, select utilities/disk utilites, select your volume from the left column, the the First Aid Tab, and run Verify Disk, if it comes uo with errors (they will be in red font) click repair disk.
If it does not come up with the error message (this disk is ok in green font) After a back up, try a reinstal of the OSX, if it comes up with red errors try running it till it comes up green.
If it still comes up with red errors, bring it in to an Apple dtore for a free evaluation. They will tell you what the issue is and price to fix.
Hpe this helps