<head runat="server">
.txtbox
{
border-top-left-radius: 20px;
border-top-right-radius: 20px;
border-bottom-left-radius: 20px;
border-bottom-right-radius: 20px;
}
</head>
and apply the css class to textbox as:
<asp:textbox id="TextBox1" runat="server" cssclass=""txtbox"" xmlns:asp="#unknown"></asp:textbox>
[Edit]Code block added[/Edit]