Try resetting your preferences as described in the FAQ.
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http://www.adobeforums.com/webx/.ef4a07f/1>
You either have to physically delete (or rename) the preference files or, if using the Alt, Ctrl, and Shift method, be sure that you get a confirmation dialog.
This resets all settings in Photoshop to factory defaults.
A complete uninstall/re-install will not affect the preferences and a corrupt file there may be causing the problem.