Mmmm.... io ho risolto così:

#mainbox {
}

#testata {
height: 200px;
overflow: auto;
}

#colsx {
float: left;
width: 20%;
height: 90%;
}


#colmain {
height: 90%;
overflow: auto;
margin: 0px 0px 0px 0px;
}

#footer {
clear: both;
margin: 0px;
padding: 0px;
}

Kisses,

Nat