hey,
you can register a client script after the login button is pressed. and show a javascript alert message.
e.g.
if (Validated)
{
}
else
{
ScriptManager.RegisterClientScriptBlock(Me.Page, Me.GetType(), "ShowError", "return InvalidLogin();", True)
}
and in the markup of your page add the script
<script type="text/javascript">
function InvalidLogin()
{
alert("Invalid username or password");
return false;
}
best of luck
Ahsan Sarfraz