Baby Smash! is a free utility that will lock your keyboard and mouse (including the Windows Key, as well as Ctrl-Esc and Alt-Tab).
You can use it also to clean them, but the original purpose of this utility is to entertain your toddler while protecting your computer.
As babies smash on the keyboard, colored shapes, letters and numbers appear on the screen.
Features: Smash on any keys; Draws letters, shapes, numbers; Keeps babies happy; Won’t harm your computer; New features added often.
Pressing ALT-F4 will exit the application and Shift-Ctrl-Alt-O brings up the options dialog.
Note that the program is unable to lock the special keys of Microsoft (and perhaps similar) keyboards.