CSS/HTML bug; help solve a "shifting" website

A website already created shifts on a few pages from side to side. Look at [login to view URL] and click around the navigation. It appears to shift from left to right, the whole website, as you click through the site. The site is 40 pages of HTML. There is existing ASP code and a PAYPAL shop which must not be damaged or ruined in any way. Just the CSS/HTML! Please, read this posting and look at the site and only respond to this project if you really UNDERSTAND the problem and KNOW how to help. I attach two versions of a random HTML page to study. When you respond please DIAGNOSE the problem and give me your (brief) PLAN how it should be fixed. Many thanks, Fiona