You don't say which web editor you are using and the methods can differ dramatically from one to another, but...
You can horizontally centre your movie by wrapping it with the <div align="center"><div> tags, but this does not account for the verticals.
Dreamweaver has horizontal and vertical spacing for Flash inserts, but it's on a specified pixel basis so may not be appropriate.
Two tricks I've successfully used are as follows:
1. Make a single celled table where you specify the page margins top, bottom and both sides. This only needs to be a couple of pixels for our purposes, but the important thing is that all margins are the same size. Setting the table cell alignment to centre (both vertically and horizontally) automatically puts any objects or text inside it bang in the middle of the page.
You can remove the table border and set it's physical size slightly bigger in pixels than the movie itself.
2. Create a simple CSS for the page or entire site where you specify the vertical and horizontal alignment for the Flash class of object and reference it in your page code.
Example code for a simple table could be:
<table width="100%" width="100%">
<td align="center" valign="middle">
< your flash code goes in here>
Finally, the simplest way of all that I know of is to download the extremely useful Flash add-in called the "Center in Browser" extension. It does what it says on the tin, takes all the headaches out of things and answers you needs regardless of which web editor you use. You can find it at the Macromedia site and it's free to download if you register with them.
Hope this helps.