The W3C or World Wide Web Consortium is the organisation that defines HTML and related standards. Their function is to provide a standardized definition which others should follow in order to make HTML interchangeable - if everyone interprets HTML different, how should that work?
However, they can´t force anyone to follow the standard - only recommend doing so. In the beginning of the web, several browsers developed their own "tags"(HTML commands) to differentiate from other browsers, trying to gain attention with funky functions like scrolling or blinking text. The idea was that designers would use them, but users of the "wrong" type of browser couldn´t see them - thus switching.
If your browser doesn´t display my site correct, it´s in most of the cases the fault of your browser. I do make mistakes too, of course, but my pages are valid HTML 4.01 (Strict).
While this does not guarantee that the site is free of errors, it means that every compliant browser *should* display it similar, which, in turn, should mean that my site looks the same on any browser.
However, this is not the case - especially with Internet Explorer, there are many problems.
One problem I just noticed is that IE apparently doesn´t treat some CSS commands as it should - the "position:fixed" to precise. So either the browser is buggy, or my site. The HTML validator says that my site is OK, as does the CSS validator.
Mozilla Firefox seems to display it allright - supposedly it is one of the most W3C compliant browsers. You might consider a switch. I do.(Update: I did. Went smooth as it could be - the design is a bit different, but one gets used to it. Didn´t have any problems with pages either.)
In the unlikely(as I like to think) case of it being MY fault - write me.
I´ve also tested my site with Opera, of course - works just as well. That´s a nice browser too, I recommend you try it, if you don´t like Firefox.
I´m not feeling right with Opera, but if there were no Firefox, it´d be my choice.