[pygame] refresh problem

[pistacchio <pistacchio@xxxxxxxxx>]

hi to all!
this is my first post in pygame (windows xp, python 2.5). i've started 
using pygame some days ago and after a bit of coding i'm completely 
stuck. the problem is that even the simpliest code like

import pygame
from pygame.locals import *
pygame.init()
screen = pygame.display.set_mode( (640,480) )

done = False
while not done:
    pygame.display.flip()
    for event in pygame.event.get():
        if event.type == QUIT:
            done = True

get the game window to freeze. it just doesn't update and acts 
strangely. the window is like "transparent", it makes me see what is 
behind it. if i minimize it and then reopen it, it shows the content, 
but if i drag it around the screen, it sort of "collects" the images it 
overlaps.. i've tried various code to get around the problem, like 
various flags in set_mode, flip and update and event pumping but nothing 
seems to work. it shows no problem in fullscreen mode, though.
any idea? thanks in advance,
gustavo