Is your hard drive very full? If so try deleting some things to make more free space (aprox 1gig).
If you have enough free space then your drive may need defragmentation. You'll need to get a utility program to do this. But before you do you want to make sure you have a good backup of your stuff. Get a Firewire external drive that is at least the same size if not bigger than your laptops hard drive. Once you have backed up you can run the defrager utility. A different approach if you don't want to buy the utility is to reformat the hard drive after making the backup and then copying back the backup files back on to your laptop. As long as you don't install a fresh OS on the internal drive this also works.
Lastly, your hard drive may be going bad at which point you'll need a new one. Consider how much stuff you have on your system now that you want to keep and how much space is left on your internal hard disk. If you find your need more space think about getting a bigger drive. But before you go there make sure you have a backup of your files.