I really don't think you quite understand this yet, do you?
Your command string sent to the mci device will evaluate to something along teh lines of:
open D:\Temp\myVideo.mp4 type mpegvideo alias mf style child parent System.Windows.Forms.PictureBox, SizeMode: Normal
since PictureBox.ToString generates
System.Windows.Forms.PictureBox, SizeMode: Normal
Whereas according to MSDN
] the parent should be a handle to a window. You could try using this.VideoOutput.Handle, as it might give a slightly better result, but I haven't tried it, and don't know if it will work.