Muchas, gracias David el Grande, sos un grande. Bueno, cualquier duda te estare consultando. Y finalmente el codigo queda asi:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
function colorear(obj)
{
fila = obj.parentNode.parentNode;
if(obj.checked)
fila.style.backgroundColor = "#FFCC99";
else
fila.style.backgroundColor = "#DDDDDD";
}
function cambiar_color_over(celda){
if (!celda.getElementsByTagName("input")[0].checked)
{ /* Cambiar color */
celda.style.backgroundColor="#ccffcc";
}
}
function cambiar_color_out(celda){
if (!celda.getElementsByTagName("input")[0].checked)
{
celda.style.backgroundColor="#dddddd";
}
}
</script>
</head>
<body>
<table>
<thead>
<tr align="center">
<th>Campo1</th>
<th>Campo2</th>
<th>Campo3.</th>
</tr>
</thead>
<tr bgcolor='#F0F0F0' onmouseover="cambiar_color_over(this)" onmouseout="cambiar_color_out(this)">
<td><input id="miCheck1" type="checkbox" name="miCheck" onclick="colorear(this)"></td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr bgcolor='#F0F0F0' onmouseover="cambiar_color_over(this)" onmouseout="cambiar_color_out(this)">
<td><input id="miCheck2" type="checkbox" name="miCheck" onclick="colorear(this)"></td>
<td> </td>
<td> </td>
<td> </td>
</tr>
<tr bgcolor='#F0F0F0' onmouseover="cambiar_color_over(this)" onmouseout="cambiar_color_out(this)">
<td><input id="miCheck3" type="checkbox" name="miCheck" onclick="colorear(this)"></td>
<td> </td>
<td> </td>
<td> </td>
</tr>
</table>
</body>
</html>