You may have something installed that the system doesn't like. Let's try a few troubleshooting steps.
First off, unplug EVERYTHING (including cell phones, iPods... everything!) from your computer except a standard keyboard and mouse. Reboot and see if you continue to have trouble. If you don't have any more trouble, then the problem has something to do with something that was connected.
If you continue to have trouble, then create a new user and log into that user. If you have no trouble with the new user, then something is rotten in your regular user's Library. If you continue to have trouble, then likely something is rotten in the root Library. Try these steps first and let us know how it works out.