I turned off cforms
Then, using plugin's editor, I have selected cforms II and changed
cforms/cforms.php
from
### PLUGIN VERSION CHECK ON PLUGINS PAGE
add_action( 'after_plugin_row', 'cf_check_plugin_version' );
function cf_check_plugin_version($plugin)
{
global $plugindir,$localversion;
if( strpos($plugindir.'/cforms.php',$plugin)!==false )
{
$checkfile = http://www.deliciousdays.com/download/cforms.chk;
to
### PLUGIN VERSION CHECK ON PLUGINS PAGE
add_action( 'after_plugin_row', 'cf_check_plugin_version' );
function cf_check_plugin_version($plugin)
{
global $plugindir,$localversion;
return;
if( strpos($plugindir.'/cforms.php',$plugin)!==false )
{
$checkfile = http://www.deliciousdays.com/download/cforms.chk;
But when clicking over "save file" button, I get an
HTTP Error 403 – Forbbiden
Don't understand why.
Then I used FTP to change /wp-content/plugins/cforms/cforms.php
It is not so smart, but it works (maybe it is the right way. I don't know)
After that, I have turned on cforms, and everything ran ok. 
Now, I have one doubt. What about future versions? Do I need to repeat the change after every cforms update?
I'm much obliged to you for cforms II and for your support.
Xavier