How can i minimize the windows form application while processing other task

Couple of bad ideas here. First, don't minimize or hide the window. Instead, perform your lengthy operation is a separate thread and leave you UI accessible, visible and responsive, only disable some controls.

Secondly, who needs MDI? Don't torture yourself and scare off your users. Do yourself a great favor: do not use MDI at all. You can do much easier to implement design without it, with much better quality. MDI is highly discouraged even by Microsoft, in fact, Microsoft dropped it out of WPF and will hardly support it. More importantly, you will scare off all your users if you use MDI. Just don't. I can explain what to do instead. Please see:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages[^],
Question on using MDI windows in WPF[^],
MDIContainer giving error[^],
How to Create MDI Parent Window in WPF?[^],
How to set child forms maximized, last childform minimized[^].

As to the threading with UI, please see my past answers:
http://www.codeproject.com/Answers/159125/Control-Invoke-vs-Control-BeginInvoke#answer1[^],
Problem with Treeview Scanner And MD5[^].

—SA