If you want to make all your tables transparent use the code style="filter:alpha(opacity=75);"
in a form like:
Code:
<table width="100%" cellpading="0" cellspacing="0" style="filter:alpha(opacity=75);">
If you want single rows or columns to have transparent use it like:
Code:
<tr valign="top" style="filter:alpha(opacity=75);">
or
Code:
<td align="center" style="filter:alpha(opacity=75);">
Keep in mind that the contents of the table will also reflect the opacity change, in other words the text will also have a transparency.