Just taking a shot in the dark here since I currently don't have IE7/8 installed, but here's my two cents:
It seems like you're targeting versions of IE lower than 9, so specifically IE7/8 using ie7.css.
Assuming you're using the standard Bootstrap repository and haven't customized it yourself.
In this scenario, the stylesheet is likely only compatible with IE7, causing issues with IE8.
You have a couple of options to resolve this:
1) Remove the existing conditional comments and create a new stylesheet for IE8 within its own set of conditional comments exclusively for IE8.
Keep the current ie7.css stylesheet and enclose it in conditional comments targeting only itself.
&@60;!--[if lt IE 9]&@62;
&@60;script type='text/javascript' src="http://html5shiv.googlecode.com/svn/trunk/html5.js"&@62;&@60;/script&@62;
&@60;script type='text/javascript' src="js/respond.js"&@62;&@60;/script&@62;
&@60;![endif]--&@62;
&@60;!--[if IE 8]&@62;
&@60;link href="/css/bootstrap-ie8.css" rel="stylesheet"&@62;
&@60;![endif]--&@62;
&@60;!--[if IE 7]&@62;
&@60;link href="/css/bootstrap-ie7.css" rel="stylesheet"&@62;
&@60;![endif]--&@62;
2). Keep ie7.css as is, add new styles for IE8 within the same file using specific hacks to target each version. You can find more information and examples of these hacks at: