Try restarting the computer.
Repaired permissions with Disk Utility?
Try starting iTunes with the option key held down and have it create a new library as a test. If it runs under these conditions then you probably have a corrupt library. In that case see: iTunes: How to re-create your iTunes library and playlists - http://support.apple.com/kb/ht1451
Try starting iTunes with the shift key (command+option on some versions) held down which disables third party plugins. If it runs under these conditions then you probably have a conflict. iTunes: Troubleshooting issues with third-party iTunes plug-ins - http://support.apple.com/kb/TS3430
There are various iTunes preferences files that can be isolated to test if they are creating the problem.