Жуть какая-то...
Как вариант...
<!DOCTYPE html>
<html>
<head>
<script src="http://code.jquery.com/jquery-latest.js"></script>
<!--
<link rel="stylesheet" type="text/css" href="tmp.css" />
-->
<style type="text/css">
</style>
<script type="text/javascript">
function calc(N) {
var val=document.getElementById('plusfield'+N).value-document.getElementById('minusfield'+N).value;
document.getElementById('fieldRes'+N).innerHTML=val;
}
</script>
</head>
<body>
<table width="70%" border="0">
<tr>
<td rowspan="2">Сотрудник</td>
<td colspan="3">Балы</td>
</tr>
<tr>
<td>+</td>
<td>-</td>
<td>Итого</td>
</tr>
<tr>
<td>Оля</td>
<form>
<td><input type="text" id="plusfield8" onkeyup="calc(8)" value="6"></td>
<td><input type="text" id="minusfield8" onkeyup="calc(8)" value="2"></td>
<td><span id="fieldRes8">0</span></td>
</form>
</tr><tr>
<td>Лида</td>
<form>
<td><input type="text" id="plusfield7" onKeyUp="calc(7)" value="5"></td>
<td><input type="text" id="minusfield7" onKeyUp="calc(7)" value="2"></td>
<td><span id="fieldRes7">0</span></td>
</form>
<script language="JavaScript">
</script>
</tr><tr>
<td>Марина</td>
<form>
<td><input type="text" id="plusfield3" onKeyUp="calc(3)" value="4"></td>
<td><input type="text" id="minusfield3" onKeyUp="calc(3)" value="3"></td>
<td><span id="fieldRes3">0</span></td>
</form>
<script language="JavaScript">
</script>
</tr>
</table>
</body>
</html>