如何使用HTML和JavaScript创建“有限修复”的div？

Given the existence of other divs on a page, how would one create a div that acts as if it's fixed-width within a certain domain of a web page? Example: the commenting system on Slashdot, which acts like a fixed-width div for purposes of scrolling along a screen but will remain within a certain length? I want a block of text to appear alongside the screen for a certain part of the page, but I want it to Given the existence of other divs on a page, ho