The simplest thing to do is to use a label control and normally it will
not be visible. I also like to set the font color to red.

<asp:Label id=errMsg runat=server fore-color=red visible=false />

Then in the code, when I detect the error,

If(thatErrorThing)

{

errMsg.Text = "Some type of message";

errMsg.Visible = true;

return;

}

in the page load event, always do

errMsg.Text = "";

errMsg.Visible = false;

But, if you WANT an alert window, you can just use a literal Web control
like so:

<asp:Literal id="ltlErrMsg" runat="server" />

And then set its Text property to the appropriate JavaScript code:

ltlErrMsg.Text = "<script language=""JavaScript"">" & vbCrLf &
"alert(""Hello, World!"");" & vbCrLf & "<" & "/script>"

Another option would be write  this

Page.RegisterStartupScript("alert_window", "<script
language=\"javascript\">alert('contact exists');</script>");