<div id="main-wrapper">
<div id="fixed-width"> lorem ipsum </div>
<div id="rest-of-space"> dolor sit amet </div>
</div>
CSS:
#main-wrapper {
100%;
background:red;
}
#fixed-width {
width:100px;
float:left
}
#rest-of-space {
margin-left:101px;
/* May have to increase depending on borders and margin of the fixd width div*/
background:blue;
}