It's not a serious problem, but whenever I try to watch a movie fullscreen, the video lags terribly. In windowed mode, it's fine. Windows Media Player doesn't have a problem with full screen, it's just iTunes. Is this just a bug?

No, it's not. Itunes uses more memory than WMP. Set the priority to Real-time on Itunes.exe (in the task manager process tab) and shut off the Vista Sidebar. This should fix the "LAGG"

i know the answer to your question. it is nothing the same thing happened to me. you need to convert that film/movie to ipod format. once you convert it it will not lagg.
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