Hola Bhonox.
Prueba asi:
Código PHP:
<html>
<head>
<script language="JavaScript">
function cambia(){
with (document.frm){
bChecked = box.checked;
if (bChecked){
send.value = "mas lineas";
lineas.disabled = false;
} else {
send.value = "enviar";
lineas.disabled = true;
}
}
}
</script>
</head>
<body>
<form method="get" action="index.php" name="frm">
<input type="checkbox" name="box" onclick="cambia()">
<input type="text" name="lineas" disabled>
<input type="submit" name="send" value="enviar">
</form>
</body>
</html>
Saludos.