IE5.5? You are sure clients still use that? Tell them to upgrade or use a different browser.
IE 5.5 is something like 10 years old, there is no excuse to be using it. It is pre-CSS, that is why is looks different. IE5.5 does not understand much CSS and no modern CSS2 and it did not exist then. Microsoft no longer supports IE5.5., it is neither updated or further brought up to speed. It is dead.
It cannot be that any developer be expected to support a dead browser. You would not expect to be able to go to Ford and have them be producing parts for a Model T would you? We waste enough time trying to get things working right in a piece of junk like IE6 to spend time on a browser that does not even support modern standards. I would literally charge them more for that kind of support.
About the only way to make a web site for IE5.5 and below is to create a second site free of standards, CSS and using tables and all the HTML Tags that the W3C has retired over the last 10 years.