Two problems with my thinbox

[zibizibi]

Hi everybody,

I have 2 problems related to a "jQuery thinbox". The first one is that, instead of the background behind the thinbox becoming transparent-gray on IE8, it becomes totally black. Here is my CSS :

#background_popup {
height: 100%;
width: 100%;
background: #000000;
position: absolute;
top: 0;
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=75)";
filter:alpha(opacity=75);
}
#popup {
border: 8px solid #353230;
position: absolute;
top: 50%;
left: 50%;
background: white;
width: 400px;
height: 200px;
margin: -100px 0 0 -200px;
}

.. On FF and Chrome, the transparency works fine, not on IE8.

My second problem is quite different. It is that, when I have a long page (several screens tall), the gray zone does not extend to the bottom of my page but only the bottom of the first screen. So if I am currently 3 or 4 screens down, I don't even notice a change since all happens far above where I am.

What can I do to fix those problems ?