The event you want is onFocus, which is when the element takes focus, of course. There are several different ways to select your text box other than clicking, such as tabbing or caret navigation.
You can also use onBlur to refill the default value if the textbox is left blank, here's an example of the two in action:
HTML Code:
<script type="text/javascript">
function blank(a) { if(a.value == a.defaultValue) a.value = ""; }
function unblank(a) { if(a.value == "") a.value = a.defaultValue; }
</script>
<input type="text" value="email goes here" onfocus="blank(this)" onblur="unblank(this)" />
Have you tried turning it off and on again?