If it is talking about Windows 2003 server, then I would assume that it would be the fastest on Windows XP. Once again it depends on the configuration of things on your/the computer. Windows 2003 server is a lot like Windows XP. In fact, like Windows 98/Windows 2000 and Windows NT 4/Windows 95, Windows XP and Windows 2003 are practically the same (not that they are the same, but they were designed to be compliments to each other and around the same time--ok, screwed up analogy). Most likely, it is the fastest on Windows XP, but one again, it depends on the configuration and if soemthing in the benchmark was specifically reliant on something that only a server can give.