SurferTim:
You don't have to answer. If you manage to get by the hardware problem, then I would suspect this comparison:

        if(readString.trim() == ("GET /?C=1678 HTTP/1.1")){

//client.println("

Clave Ok

");
         Portada();
      }else{
         //client.println("

Clave Erronea

");
      }

Just talking out loud here. I would try this instead. But that is just me.

if(strcmp(readString.trim(), "GET /?C=1678 HTTP/1.1") == 0) {
         //client.println("

Clave Ok

");
         Portada();
      }else{
         //client.println("

Clave Erronea

");
      }

i try this but gives errores

 if(strcmp(readString.trim(), "GET /?C=1678 HTTP/1.1") == 0) {
          //client.println("<h1>Clave Ok</h1>");
          Portada();
       }else{
          //client.println("<h1>Clave Erronea</h1>");
       }

look:

http://www.freeimagehosting.net/qap5b