Javascript OnkeyPress not working in TabContainer Control

Hi,

           I have tab container control.Inside the tab panel i have lot of textboxes.I used  javascript validation not to enter alphabets in textbox.But javascript not working when textbox was inside the tab.Find the code below:

<asp:TextBox ID="txtnoofpages" runat="server" Enabled="false" Height="23px" Width="200px" onkeypress="javascript:return isNumber (event)"></asp:TextBox>  

    function isNumber(evt) {
        var iKeyCode = (evt.which) ? evt.which : evt.keyCode
        if (iKeyCode != 46 && iKeyCode > 31 && (iKeyCode < 48 || iKeyCode > 57))
            return false;

        return true;
    }     

Pls help me in finding solution to this problem.Thanks in Advance