When you use blend on a HP computer, it could happen that your designer can't show any XAML files. You always get an error like 'The name "..." does not exist in the namespace
"http://schemas.microsoft.com/winfx/2006/xaml/presentation"'. After searching for quite a while I discovered this was due to an environment variable. Some HP systems set a PLATFORM environment variable to e.g. MCD or HP. This causes MSBuild (and for that matter also blend and Visual Studio) to use that platform as the target platform in stead of 'Any CPU'. Remove the environment variable and the problem should be resolved.