How to Centralize a Float on the Screen (such horizontally as vertically) Using CSS

good afternoon,

I'd like to create an area placed in the center of the screen using floats with fixed values for both width and height. this area must be centralized even when the screen is resized. my objective is to centralize on the screen the field Password, the buttom Login and an image. these elements must be placed within a float which must be centralized on the screen.

I'd appreciate your help.

thanks,