The quality and buffering requirements of streamed movies depend on your available bandwidth, (Internet connection speed, and what's available), local network set-up and Windows Media Player settings.
Movies are encoded for specific connection speeds: 512Kbps, 1Mbps, 1.5Mbps and 2Mbps, (up to the bit rate of the original content). Windows Media Player automatically detects your bandwidth and selects the most appropriate stream to playback. Sometimes it gets this wrong.
When playing the movie check the Statistics dialogue box via View - Statistics. You will see what version (stream) of the movie your Windows Media Player has selected. If it is greater than your bandwidth then you should configure your connection speed manually via Tools - Options - Performance.
When you re-stream the movie it will select a lower quality version and should not buffer during playback.
You should also ensure that no other programs/computers are consuming your Internet connection, such as Limewire, eDonkey, and various torrent and peer-to-peer utilities. It's common to believe your broadband connection is faster than it actually is. Have you tested your speed?
The other thing to watch for is wireless networks. Whilst you might have a 54Mbps wireless connection, are you actually connected at that speed? Wireless bandwidth is shared with all people in your wireless reception area so if your neighbour is downloading from iTunes on his wireless network it may well affect your wireless bandwidth. If you can, always connect to your router/modem using a cable.
If problems persist and if the option is available, download the movie in full and play it back from your hard disk.