How to Prevent Enter Key for TextBoxes

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" Runat="Server">
    <script src="Scripts/validation.js" type="text/javascript"></script>
    <script language="javascript"  type="text/javascript">
     


       
        function EmpHelp() {

            popup = window.open('EmpMstrHelp.aspx', null, 'height=320,  width=320,status= no, resizable= no, scrollbars=no, toolbar=no,location=center,menubar=no, top=100, left=300');
            common = "EmpHelp"
            return false;

        }
        function EmpId() {
           // alert('ss');
       //  return (event.keyCode!=13);
         var key = event.keyCode;
        // alert(key);
         if (key == 113 && key != 13) {
             document.getElementById('<%=btnEmpHelp.ClientID %>').click();
         }
         else {
           //document.getElementById('<%=txtEmpId.ClientID %>').focus();//its also not working and i want for all text box not for only specific one 
             //return;
         //I Want my focus should not lost and page load event should not call if Enter pressed 
          
            // return false;
         }
        }
        
        
 </script>  
    
    
    
<table>
    <tr>
        <td align="right" style="color: #000000; font-weight: bold">
            <span style="color:Red">*</span>Employee Id
        </td>
        <td align="left" style="color: #000000; font-weight: bold">
                   
            <asp:TextBox ID="txtEmpId" runat="server" Width="50px" MaxLength="5" TabIndex="1" 
                    AutoPostBack="True" ontextchanged="txtEmpId_TextChanged"></asp:TextBox>

                
            
           
             <asp:Image ID="btnEmpHelp" runat="server" ImageUrl="~/help.gif" Width="20px" />
            Search
        </td>
    </tr>

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            clear();//my fuctn that clear all values 
           
            txtEmpId.Attributes.Add("onkeypress", "javascript:return alphanumeric(event);"); //lets say i have preform some validations
           
            txtEmpId.Attributes.Add("onkeydown", "EmpId()"); //one pop up help aapers 
           
            btnEmpHelp.Attributes.Add("onclick", "EmpHelp()"); //that help button
           
        }

      
    }

  protected void txtEmpId_TextChanged(object sender, EventArgs e)
    {
      //lets say my Populated code
    }