help me in design aa login page using ajax

wat i need to design is when somebody comes to my home page there should be a login page in a semitransparent png(similar to lightbox), below that lies my home page .if login successful then the semitransparent login page disappears showing my home page .i use php my sql for login .if its possible to achieve this effect.
if so explain me in detail since am newbie . link to tutorial or scripts is greatly appreciated.


thanks in advance..

if you don't mind frameworks, check out example 7

http://prototype-window.xilinus.com/samples.html

please some one suggest please
urgent!!