Problem with corrupted display while editing on some machines

Hi We have a problem with a wiki that we are working on. It's a trivial wiki and it's hosted by the nice xwiki people (so it should not be an installation / configuration issue). The problem is that when we edit a page on some machines (but only some of them, editing the same page on another machine is usually OK) we get a corrupted display. This is happening with IE7 and Firefox 2. What might be causing this? Where do we start looking? Please see the attached image. Tim