Hide a DIV by using Javascript / CSS / AJAX

I have very small problem to fix for experts. In a social networking website when you logged in, there is a popup window opens up when page load. (It is a DIV layer - not popup window). PROBLEM: When I try to close this popup window(DIV layer), it doesn't close. The tricky point is when you click the close button, it should send a post to a Ajax page and it changes the session for this layer. It means when you close it, it should be closed when if you switch the pages as long as you click again to open Popup layer. Same session history for opened popup. NOTE: I can explain better and it will not take more than 10 minutes to fix it for any web expert who knows Ajax PHP session and CSS. The fast communication is important, I need this to be fixed by today! Thanks, Ozkan