Go into "Menu" > "Option" > "Cheats", or type these codes during the game.

Effect	Code
All secret keys found	KEYFOUND
Disable Karts-mode	KARTSOFF
Enable karts-mode	KARTSON
Reset all FMV	RESETFMV
Reset all secret keys	RESETKEY
Reset everything to default settings	RESETALL
Reset track and lap records	RESETLAP
Unlock all FMV	CINEMAON
Unlock everything	GIMMEALL