create another javascript function for the reset to clear all the fields and in onclientclick call that function ..
function clear()
{
document.getElementById('<%=txtbox1.ClientID%>').value="";
document.getElementById('<%=txtbox2.ClientID%>').value="";
document.getElementById('<%=txtbox3.ClientID%>').value="";
document.getElementById('<%=txtbox4.ClientID%>').value="";
}
this is reset button in this way you can call
<asp:imagebutton id="imgbtnReset" runat="server" imageurl="~/images/reset_btn.gif" xmlns:asp="#unknown">
OnClick="imgbtnReset_Click" OnClientClick="javascript:return clear();" />
</asp:imagebutton>