In the html file created by flash add a div tag around the object and embed tags for the movie and give this div tag an id attribute with the value my-movie,
e.g.
<div id="my-movie">
the object and embed tags here
</div>
then somewhere between the opening <head> and closing </head> tags add the following css,
Code:
<style type="text/css">
#my-movie {
position: absolute;
left: 50%; top: 50%;
margin: -200px 0 0 -275px; padding: 0;
}
</style>
the values for the margins depend on the size of the movie, if you changed the size of the movie these would need to be updated. the values should be,
margin: -(half the height of the movie)px 0 0 -(half the width of the movie)px;