how do i fix this problem - IndentationError?

[GrandMaster101]

I have never does any type of coding before and wanted to give it a try. I am using the book called python hunting. they told me to type this codes first so that I can load up pygame and eventually start developing games.

import pygame, sys
from pygame.locals import *
pygame.init()
screen = pygame.display.set_mode((640,480))
while 1:
    for event in pygame.event.get():
	    if event.type == pygame.QUIT:
		    sys.exit()
	screen.fill((255,255,255))
    pygame.draw.circle(screen,(0,255,0),(100,100),20)
    pygame.display.update()

But whenever I try to open this file on python itself common Prompt(CMD) keeps telling me

line 9
screen.fill((255,255,255))

IndentationError: unindent does not match any outer indentation level

what does this mean???? how do I correct this???? Can someone please help. I have been trying to creak my head on this for more than 1 month now. A help will be really appreciated. Thankyou.