[Javascript] variable DIV

[vkocher at wombat.ch]

Hello
 i have a page with 3 column  the Middle and right one have a fixed 
width the Left one has a Variable Width, as large as the Browser/Screen 
would be.

It is possible to read it out with javascript how large the 
Screen/Browser is deduct the px from the Middle and right Column and add 
the Result to a DIV Structure?

and how to do it?
the place this should work would be http://jetztunterwegs.blogspot.com/
and sorry i am a beginner withe javascript - so the how to do should be 
easy to add to my page

thanks a lot
have a nice day

vincent