It's your widths in the table at the top of the right column for the no. of products in category, etc. You have a table with a width of 430px. In your second row of that table, you have a nested table also with a width of 430px. But, since your image spans the first cell in the second row, that nested table is wider than the parent table, which is pushing the design too wide for the right column in IE6, and forcing it to drop below to accommodate the extra width.
<table align="center" border="0" cellpadding="0" cellspacing="0" width="430">
<tbody><tr>
<td rowspan="4" valign="bottom" width="50"><div style="padding: 10px;"><img src="graphics/00000001/PW-SJ.jpg" alt="Sewer Jetters & Accessories" border="0"></div></td>
<td style="color: rgb(82, 82, 74); font-size: 12px; text-decoration: none;" valign="bottom" width="190">
Result pages:
<font color="#990000">[</font>1<font color="#990000">]</font>
Page 1 of 1
</td>
<td valign="bottom" width="190"><table align="center" border="0" cellpadding="0" cellspacing="0">
<tbody><tr>
</tr>
</tbody></table></td>
</tr>
<tr>
<td colspan="2" align="left" valign="bottom">
<table align="center" border="0" cellpadding="0" cellspacing="0" width="430">
<tbody><tr>
<td style="color: rgb(82, 82, 74); font-size: 12px; text-decoration: none;" valign="bottom" width="380">Products found in this category: <b>24</b></td>
</tr>
</tbody></table></td>
</tr>
</tbody>
</table>
Just delete the width from that nested table -- you don't need it.
That'll be $500, please :)