How to validate a field and not allow " to be entred by user

I have this code:
<tr>
                <td colspan="2" style="color:#555555; width: 336px; height: 46px;"
"font-family:Verdana,Tahoma,arial,sans-serif;"
"font-size:70%;"
"font-size-adjust:none;"
"font-style:normal;"
"font-variant:normal;"
"font-weight:normal;" align="left" valign="top">
                Additional comments:<br />
                <asp:textbox ID="txtComment" runat="server" MaxLength="255" Width="466px"></asp:textbox>
                <!--<asp:textbox ID="Textbox1" ru ...