When I read my colleague Adrian Kingsley-Hughes’ epic account of his benchmark tests of Windows Vista SP1 versus Windows XP SP2, the first thing that struck me was how far apart his numbers were from those I was seeing. In fact, I went back and redid all my tests to confirm that I hadn’t missed anything along the way. They checked out completely. On my test bed, with only one exception, Vista SP1 was consistently as fast as or faster than XP SP2. Why the difference? I have a few theories.