colspec attribute getting ignored

Hi,
I am quite new to using docbook, and im trying to set up documentation for a
piece of software. I am using XMLMind XML editor to create the file, and had
to insert the colspec attribute by hand (good ol' notepad) after I could not
find it in the editor. It does recognise the colspec attribute in the editor
as it has resized the columns as I want them, but whenever I compile it into
the online help file, the column size is not as specified. Any help or
thoughts are appreciated.

Felix

Below is the code as seen in notepad.

<informaltable  frame="none">
<tgroup align="center" cols="2">
<colspec colwidth="4cm"></colspec>
<colspec colwidth="8cm"></colspec>
<tbody>
<row>
<entry align="center" valign="middle">Button</entry>
<entry align="center" valign="middle">Function</entry>
</row>
<row>
<entry><graphic align="center" fileref="toolopen.png" format="PNG"
align="middle"></graphic></entry>
<entry>This button is to open a .log file from a selected source.</entry>
</row>
</tbody>
</tgroup>
</informaltable>