It could be many things including a defective hard drive.
General Mac maintenance: Tips to keep your Mac in top form
General purpose Mac troubleshooting guide: Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account
You could also try
OnyX for Mac
If none of this helps post back
I provided many things to do and look at some may cure your issue, let's not jump to conclusions. Do some of the tests, run the ONYX Application it will verify the status of your hard drive and then we can look at other things if the issue isn't already resolved. Make sure you have a backup in place, now, as your data and applications may be at risk.