This article presents results of an investigation of the usage of .NET on five versions of Windows. The operating system files for the first version of Windows tested, XP Pro with Service Pack 2 applied, did not use .NET at all. This is understandable because XP was released before .NET was first released. The next version of Windows was the PDC 2003 build of Longhorn. This has a similar number of unmanaged executable files as XPSP2 but it also had thirty five .NET assemblies. Amongst these assemblies were two services.