100% vertical, but not horiz DIV Box?

Question

Hopefully I can describe this well enough to be able to get
some help, fingers crossed!
You can see one of the websites using the layout I'm having
problems with here;
http://proceduresonline.com/bexley/chapters/values.html
(please bear in mind they aren't meant to be pretty or flashy
as they're simplified online manuals, essentially)
OK so,
The layout is a fixed size as the original layout, which
filled 100% of screens (as in it fitted automatically to your own
screensize) meant that people with large resolutions had far too
much text horizontally, making it hard to read, but people with
smaller resolutions were having trouble with elements overlapping
and missing buttons and so on.
So this one is fixed at such a size as to fit perfectly in a
1024 screen, but that does mean a lot of dead space on higher
resolutions.
The layout is just made up of three div areas; the container
to hold it all, one left-hand column, and one right-hand column
which holds the content of each page.
Here's the container code
.twoColFixLt #container {
width: 780px;
margin: 5px auto;
text-align: left;
Here's the left side code
.twoColFixLt #sidebar1 {
float: left;
width: 150px;
background: white url("../required
parts/side-shadow-alt.png") no-repeat bottom right ;
padding: 10px 25px 10px 20px; }
Here's the right side code
.twoColFixLt #mainContent {
background: white url("../required
parts/content-shadow-alt.png") no-repeat bottom right;
margin: 0 0 0 210px;
padding: 10px 10px 25px 15px;
In that right hand column is a very simple styled div box
which makes all the text in it display as a scrollbox rather than
force the page off the screen; <div
style="width:100%;height:500px;overflow:auto;border-width:0px;">
What i'd like to be able to do is to allow the right-hand
column (the one which holds the content) to fit 100% to a persons
screen.
I have tried various methods to make this section fit 100%,
but no matter what I try that div-box-scroll-bar part refuses to
accept percentage values. If its not got a fixed-height value it
just displays all of the contents, and so loses its scroll bar. and
this also forces the right-hand column that it sits in to go off
the page too, giving the whole page a scrollbar, which is what I
want to avoid.
What annoying is there its only that scrolly-div that has any
height value! none of the containers do, so presumably all I need
to do is to get that to fit and it'd work?
Sorry, II know this is a bit of a bad explanation - any
ideas?

Answer 1

I wish I could explain this better, but that will not help as I need the height to be dependant on the 100% width. Here is a sample of the problem in action:
http://www.johnkalnin.com/flash/banner.html
See how you have to scroll down to see the content.
As you can see the 100% width of the movie is messing up the height of the containing div.