how to center the div at the screen?

hi all,

My page have vertical scrollbar and i have some buttons in somewhere on the page. When i click that buton i'm showing one div. so now i need to show that div at the center of the screen. But if scrolled down at the bottom of page then if i click the buton at bottom then it should show at center of screen, similarly if i'm at middle of the page or at top. So please help me how to display the div at center of the page using div style.

Thanks! ...