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Normally you do NOT want to block operating system key combinations but there are a few legitimate cases where you do. In my case I am making a pygame script for my 1 year old to bang on the keyboard and see/hear shapes/color/sounds in response. Brian Fischer on the pygame mailing list pointed me to pyHook. This example was taken from here: http://www.mindtrove.info/articles/pyhook.html and modified to use the pygame event system.

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import pyHook
import pygame

# create a keyboard hook
def OnKeyboardEvent(event):
	print 'MessageName:',event.MessageName
	print 'Message:',event.Message
	print 'Time:',event.Time
	print 'Window:',event.Window
	print 'WindowName:',event.WindowName
	print 'Ascii:', event.Ascii, chr(event.Ascii)
	print 'Key:', event.Key
	print 'KeyID:', event.KeyID
	print 'ScanCode:', event.ScanCode
	print 'Extended:', event.Extended
	print 'Injected:', event.Injected
	print 'Alt', event.Alt
	print 'Transition', event.Transition
	print '---'
	if event.Key.lower() in ['lwin', 'tab', 'lmenu']:
		return False	# block these keys
	else:
		# return True to pass the event to other handlers
		return True

# create a hook manager
hm = pyHook.HookManager()
# watch for all keyboard events
hm.KeyDown = OnKeyboardEvent
# set the hook
hm.HookKeyboard()

# initialize pygame and start the game loop
pygame.init()

while(1):
	pygame.event.pump()

pyHook does not block the ctrl-alt-del combination even if you try to make it. (which is a good thing) But in my case I can now easily block the windows key and the alt-tab combination. I plan to just use the hook to block a few events and do my regular event processing after the pygame.event.pump() call, like any normal pygame loop.

UPDATE 09/2011 I've added this same example to my blog, where I intend to post future recipes and discussions. http://fadedbluesky.com/2011/using-pyhook-to-block-windows-key/