Well at least that makes sense--I had never heard of anything like this on XP. We are working on a fix for this for Vista. In the mean time, all I can suggest is to either run it on XP or split the experiment as described above. As of this time I do not know the extent to which this is happening across Vista machines. I can at least say we have had many more requests for Vista authorization codes that we have had reports of this problem. But then that may simply be due to the fact that most experiments maybe be shorter than 150 items, or that the problem is not be reported by most users.