ciao

non indicare nei TD colspan la larghezza della cella
non ha senso....

codice:
<table width="550" border="1" cellspacing="0" cellpadding="0">
  <tr>
    <td colspan="2" align="center" valign="middle"><div align="center">A</div></td>
    <td width="16"  align="center" valign="middle"><div align="center">B</div></td>
    <td colspan="2" align="center" valign="middle"><div align="center">C</div></td>
  </tr>
  <tr>
    <td width="100" align="center" valign="middle"><div align="center">D</div></td>
    <td colspan="3" align="center" valign="middle"><div align="center">E</div></td>
    <td width="100" align="center" valign="middle"><div align="center">F</div></td>
  </tr>
  <tr>
    <td colspan="5" align="center" valign="middle"><div align="center">G</div></td>
  </tr>
</table>
ciao Max