To 'Reset' Sibelius and the Kontakt Player to the way it was when you first installed it you should:

On Windows:
• Close Sibelius - you may need to end the task by doing Ctrl+Alt+Delete
• Download this Registry key, and save it to disk, e.g. on your Desktop
• Double-click the downloaded Registry file
• When asked if you want to add the contents of the key to your Registry, click Yes
• Click OK when informed that the contents of the key were successfully added.
• Start Sibelius again and this time Sibelius should start up with no problems. You will notice however that such preferences as the Kontakt Player's registration (which you don't register again if you have already registered the Kontakt Players) and the Playback and Input devices will need to be set again.

On Mac:
• Close Sibelius - you may need to Force Quit it by doing Apple+Alt+Escape.
• Go to HardDrive > Users > Your Username > Library > Prefernces
• Drag into the Trash com.sibelius.sibelius4.plist and siblib.plist
• Empty the Trash
• Start Sibelius again and this time Sibelius should start up with no problems. You will notice however that such preferences as the Kontakt Player's registration (which you don't register again if you have already registered the Kontakt Players) and the Playback and Input devices will need to be set again.

NOTE: This will only effect Sibelius and the Kontakt Players and has no affect on any other program