Переименуйте функцию
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>Калькулятор</title>
<style type="text/css">
#calc{
background:#CCC;
border: groove 7px;
border-radius: 10px;
width: 160px;
height: 200px;
margin-top: 10px;
padding: 2px;
display:block;
}
#pole{
background: #777;
width: 148px;
height: 30px;
margin: 5px 5px 10px;
}
#disp {
margin-top: 4px;
margin-left: 3px;
text-align:right;
}
.knopka {
width: 30px;
height: 30px;
margin: 2px 5px 5px;
float:left;
}
</style>
</head>
<body>
<div id="calc">
<form action="" name="calcjs">
<div id="pole"><input type="text" name="disp" id="disp" value="0" size="18" maxlength="10"></div>
<div class="knopka"><input type="button" value="7" width="30" height="30" id="k7"></div>
<div class="knopka"><input type="button" value="8" width="30" height="30" id="k8"></div>
<div class="knopka"><input type="button" value="9" width="30" height="30" id="k9"></div>
<div class="knopka"><input type="button" value="C" width="30" height="30" id="kc" onclick="res()"></div>
<div class="knopka"><input type="button" value="4" width="30" height="30" id="k4"></div>
<div class="knopka"><input type="button" value="5" width="30" height="30" id="k5"></div>
<div class="knopka"><input type="button" value="6" width="30" height="30" id="k6"></div>
<div class="knopka"><input type="button" value="/" width="30" height="30" id="kd"></div>
<div class="knopka"><input type="button" value="1" width="30" height="30" id="k1"></div>
<div class="knopka"><input type="button" value="2" width="30" height="30" id="k2"></div>
<div class="knopka"><input type="button" value="3" width="30" height="30" id="k3"></div>
<div class="knopka"><input type="button" value="*" width="30" height="30" id="ku"></div>
<div class="knopka"><input type="button" value="0" width="30" height="30" id="k0"></div>
<div class="knopka"><input type="button" value="=" width="30" height="30" id="kr" onClick="res()"></div>
<div class="knopka"><input type="button" value="+" width="30" height="30" id="kp"></div>
<div class="knopka"><input type="button" value="-" width="30" height="30" id="km"></div>
</form>
</div>
<script>
function res() {
document.calcjs.disp.value = "";
alert("111");
}
</script>
</body>
</html>