Código HTML:
Ver original<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <script type="text/javascript"> function enviar(){
document.form1.button.disabled = true;
for (i=0;i<document.form1.elements.length;i++){
if(document.form1.elements[i].type == "checkbox"){
if(document.form1.elements[i].checked == 1 ){
document.form1.button.disabled = false;
i=document.form1.elements.length+10;
}
}
}
}
<form id="form1" name="form1" method="get" action=""> <input name="campos[]" type="checkbox" id="checkbox" value="1" onchange="enviar();" /> <input name="campos[]" type="checkbox" id="checkbox2" value="2" onchange="enviar();" /> <input name="campos[]" type="checkbox" id="checkbox3" value="3" onchange="enviar();" /> <input name="campos[]" type="checkbox" id="checkbox4" value="4" onchange="enviar();" /> campo 4
<input type="submit" name="button" disabled="disabled" id="button" value="Enviar" />
Listo :)