Solution for the code you posted:
.center{
position:absolute;
width:780px;
height:650px;
left:50%;
top:50%;
margin-left:-390px;
margin-top:-325px;
}
<table class="center" width="780" border="0" align="center" cellspacing="2" bordercolor="#000000" bgcolor="#FFCC66">
<tr>
<td>
<table width="100%" border="0">
<tr>
<td>
<table width="100%" border="0">
<tr>
<td width="150"><img src="images/banners/BAX Company.jpg" width="149" height="130" /></td>
<td width="150"><img src="images/banners/BAX Location.jpg" width="149" height="130" /></td>
<td width="300"><img src="images/banners/Closet.jpg" width="300" height="130" /></td>
<td width="150"><img src="images/banners/BAX Company.jpg" width="149" height="130" /></td>
<td width="150"><img src="images/banners/BAX Location.jpg" width="149" height="130" /></td>
</tr>
</table>
</td>
</tr>
<tr>
<td>
<table width="100%" border="0">
<tr>
<td width="150"><img src="images/banners/BAX Company.jpg" width="149" height="130" /></td>
<td width="150"><img src="images/banners/BAX Location.jpg" width="149" height="130" /></td>
<td width="300"><img src="images/banners/Closet.jpg" width="300" height="130" /></td>
<td width="150"><img src="images/banners/BAX Company.jpg" width="149" height="130" /></td>
<td width="150"><img src="images/banners/BAX Location.jpg" width="149" height="130" /></td>
</tr>
</table>
</td>
</tr>
</table>
--
How this works?
Example: http://jsfiddle.net/953Yj/
<div class="center">
Lorem ipsum
</div>
.center{
position:absolute;
height: X px;
width: Y px;
left:50%;
top:50%;
margin-top:- X/2 px;
margin-left:- Y/2 px;
}
To position the div
vertically and horizontally, divide X and Y by 2.