Microsoft Office applications try to manage resources, processes on its own, so Process.Start() will return valid process id only if there are no other instances of MS office running. In other case Process.Start() call will return immediately without returning valid process id, because existing instance of MS office application will take over process creation and opening the file. To avoid this, special DDE switch has to be passed to MS Office application when it is started. Class below customizes starting of MS Office application. see: http://support.microsoft.com/kb/210565 for command line switch and see: http://social.msdn.microsoft.com/Forums/hu-HU/csharpgeneral/thread/21014f97-1732-4261-afe5-2127b2e64f68 for

public class MsOfficeExternalPreviewApplication : GenericExteralPreviewApplication

Public Class MsOfficeExternalPreviewApplication
	Inherits GenericExteralPreviewApplication

public ref class MsOfficeExternalPreviewApplication : public GenericExteralPreviewApplication

type MsOfficeExternalPreviewApplication =  
    class
        inherit GenericExteralPreviewApplication
    end