If you are working on webapplication (I assume its .net), then you can use javascript function as below
<script type="text/javascript">
function blanktextbox() {
var textbx = document.getElementById('<%=TextBox1.ClientID%>');
if (textbx.value == '0.00')
textbx.value = '';
}
function filltextbox() {
var textbx = document.getElementById('<%=TextBox1.ClientID%>');
if (textbx.value == '')
textbx.value = '0.00';
}
</script>
And in HTML your textbox will be
<asp:textbox id="TextBox1" runat="server" onfocus="blanktextbox();" onblur="filltextbox();" xmlns:asp="#unknown"></asp:textbox>
and you are done :)