How to open a new window or user control through command

Hi,

 

In my WPF application, I have a parent window Main.xaml where i have a button. On click of button i want to open a new window ChildWindow.xaml through command (and not through click event in code behind) 

when the child form open i want to disable the Main.xaml window so that user cannot click anything on it.

 

i need it through MVVM so creating  ChildWindow instance in the viewmodel i am not looking for. Can anybody please help me out.

 

 

Thanks.