This page demonstrates how to center text smack dab in the middle of the screen. The idea is to enclose your page text and graphics within a single-celled table, and then use the VALIGN=MIDDLE and ALIGN=CENTER attributes to center everything within that cell. Here's the basic structure:
<!--Set up a table for the entire window-->
<TABLE WIDTH=100% HEIGHT=100%>
<TR>
<TD VALIGN=MIDDLE ALIGN=CENTER>

<!--The real page text and stuff goes here-->

<!--Close the big table-->
</TD>
</TR>
</TABLE>