Here is the hacky way to do it:

Code:
<html>

<head>
  <title>My Webpage</title>
  <!-- Other header stuff here -->
</head>

<body>

  <table width="100%" height="100%">
    <tr>
      <td align="center" valign="middle">

        <table width="800" height="600">
          <tr>
            <td>

              <!-- Put your page content here -->

            </td>
          </tr>
        </table>

      </td>
    </tr>
  </table>

</body>

</html>
So, essentially what you're doing is creating a single-celled table inside a bigger single-celled table which fills the screen and the content of which is centred both horizontally and vertically.

This is a really horrible way of doing this and you can do it much neater with CSS, but this runs straight out of the box.