How to refresh or flush the Windows C # form?

This is my Entry Form of my Application.

While Clicking the OK button, It will go to next form for further processing.My Second form is

In the second Form, I have to choose,any one option button and then press ok button.After pressing ok button,i just calling some function and then it will return to Form1.But Form1 has not getting its control for some seconds.It looks like getting collapsed and needs to be refreshed.but i don't know how to refresh a form? Please Guide me to get out of this issue...

It looks like,

call this.Invalidate() or this.Refresh() on the form to update it.