Page shifts Up and loses top part of header

When I view my main page, it looks fine.
It is centered on the screen with some background space
between the top of the screen and the top of the web page.
And has background space to the left and right.

But when I select the sign-up link which is basically:
http://www.website.com/index.php?command=signup_page

When the sign-up page is viewed, it has moved up to the top of the screen
and loses the top part of the header border.

How can I get it to view exactly like the home page?

To experiment I went to the css file and changed the top margin from 50px to 500px. It only effected the home page, didn't effect the sign-up page at all.

body {
	background-color: #6B6B6B;
	text-align: left;
	margin-top: 500px;

Code (markup):

What do you suggest I do to keep the top of the sign-up page from being cut off? Thanks.

 

example???

 

Is the Sign up page a new page or just a DIV layer inside the main page?

 

thanks for your reply.

Sign-up page is a separate page