1) Check your available disk space (must be 10% or more).
You can check this by right clicking your MacIntosh HD and info, then the discovery triangle, so it faces down, then you'll see the capacity and how much is available.
2) Check your disk, by inserting the OSX install disk, press C when booting your Mac.
Then when the menu bar appears choose Utilities > Disk Utility, click First Aid, and then click Repair Disk.
Make sure that when finished you select your usual startup disk (MacIntosh HD)
3) If you don’t have a Mac OS X Install disc, restart your computer while holding down the Shift key. As your computer starts up, it performs a disk check similar to the one Disk Utility performs.
Post back your results....
SM