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.