Make sure you have installed the latest free Mac OS X updates (open software update).
If it still continues:
Backup your home folder to another place, and delete your home folder.
Next time you log in Mac OS X will create your home folder again, and copy all essentials from your old home folder such as documents, your address book list and saved e-mails (the last two live in your Library folder).
If problems still continue:
Back up all the data you wan to keep (onto a CD or another Hard Drive), format the Hard Drive and reinstall Mac OS X.