I'm not sure if this is what's causing your problem, but we had trouble with IE8 styles rendering differently than IE7 and IE6. When I searched for "IE8 css rendering" I read through many blogs. What I found is that IE8 has issues with (standard W3C) styles in .css files (
http://forum.x-cart.com/showthread.php?t=49672). If you go to msn.com or microsoft.com and right click to view the page source, you'll find microsoft has trouble with IE8 for their webpages, so they have added this hack:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
This hack forces IE8 to use IE7 mode.