function CheckAlphabets(e) {
var whichCode = 0;
if (window.event)
whichCode = window.event.keyCode
else if (e)
whichCode = e.which;
if ((c>=65 && c<=90) || (c>=97 && c<=122) || (c==32)) {
return true;
}
else {
return false;
}
}
<asp:TextBox ID="txtName" onKeyDown='javascript: return CheckAlphabets(event);' runat="server" />