You will often see the moderators of this forum constantly going on about DTDs and Doctypes, and their completeness. We don't do this for a laugh, Doctypes are important.
IE has two rendering modes - Quirks mode and Strict mode. In Quirks mode, IE renders the page however it feels like on that particular day. You will notice problems with your CSS layouts that woudln't appear in Strict Mode.
To enable Strict Mode you must use a COMPLETE DTD.
Please, ensure you have a complete and correct doctype before posting a question, as it cans olve many problems, and saves us having to waste time forcing people to use them before getting on to their problem.
DOCTYPES THAT WORK
Also, ensure that NOTHING comes before the doctype. The XML prolog that is often added by WYSIWYG editors throws IE into Quirks Mode. The DTD should be at the top of a HTML file.