I'm just guessing about how the message in the channel is formatted, but this should do it (untested).

Code:
on ^*:text:*[PRE]*:#chan: {
  if ($active == @Messages) { return }
  if (!$window(@Messages)) { window -k0n +l @Messages 50 50 500 360 @Messages Tahoma 11 }
  if ($active !ischan) {
    echo -at 8 $+ $1-
    haltdef
  }
  echo $color(normal) @Messages  $+ $nick $+  - $+ $chr(155) $network $chr(92) $chan $chr(92) $time $chr(92) $asctime(dd/mm) $+ - $+ $asctime(yy)
  echo $color(normal) @Messages $1-
  echo $color(normal) @Messages -----------------------------------
  haltdef
}


You can try this and then be specific about what it does that it shouldn't and/or what it doesn't do that it should. Also, an example of the PRE line would help.