Nothing to do with MySQL. This is exclusively a PHP question.
The answer is the same if no database is involved.
$cnotes = $row["CNotes"];
echo "<td width=20%>" . substr($cnotes,0, 100);
if ( strlen($cnotes) > 100 )
echo '<a href="showMore.php?id=' . $row["ID"] . '">... CLICK HERE TO VIEW MORE</a>';
That shows having the user open a separate page to "view more" (using a hypothetical ID field from your DB table).
EDIT: Corrected the error iBall showed. But I don't expect the above answer to be used, anyway. Almost surely you are going to want a better answer. But it's hard to give that without knowing WHAT you want. Oh, and I should have given the caveat that I don't code in PHP. If you had asked for a MySQL answer, I think I would get that right.