Press all modifiers (Alt + Ctrl + Shift) while starting Photoshop CS4 and later or delete preferences file:
Vista & Windows 7 or 8
Users/[Username]/AppData/Roaming/Adobe/Adobe Photoshop [verzija]/Adobe Photoshop [verzija] Settings\Adobe Photoshop [verzija] Prefs.psp
Windows 2k/XP
Documents and Settings\[username]\Application Data\Adobe\Adobe Photoshop [version] \Adobe Photoshop [version] Settings\Adobe Photoshop [version] Prefs.psp
For example:
C:\Documents and Settings\Bojan\Application Data\Adobe\Adobe Photoshop CS6\Adobe Photoshop CS6 Settings\Adobe Photoshop CS6 Prefs.psp
Mac
/Users/[Username]/Library/Preferences/Adobe Photoshop CS6 Settings/Adobe Photoshop CS6 Prefs.psp