Validation for simple Registration form for beginners

<script type="text/javascript" language="javscript">
        function validreg() {
            var fname = document.getElementById("txt3").value;
            var lname = document.getElementById("txt4").value;
            var emid = document.getElementById("txt5").value;
            var cnty = document.getElementById("countryid").value;
            var reason = document.getElementById("txt6").value;
            var uname = document.getElementById("txt7").value;
            var pswd = document.getElementById("pswd2").value;
            var rpswd = document.getElementById("pswd3").value;
            //alert(cnty);

            if (fname != "" && fname != null && lname!="" && lname!=null && emid!="" && emid!= null && cnty!=0 && reason!="" && reason!=null && uname!="" && uname!=null && pswd!="" && pswd!=null && rpswd!="" && rpswd!=null) { 
                var atpos=emid.indexOf("@");
                var dotpos=emid.lastIndexOf(".");
                if (atpos < 1 || dotpos < atpos + 2 || dotpos + 2 >= emid.length)
		{
                    alert("Not a valid e-mail address");
                    return false;
                }
                if (pswd != rpswd) 
		{
                    alert("Password not matched");
                }
               
                else {
                    alert("U registered successfully");
                }
            }
            else{
                alert("enter all mandatory fields");
            }
        }
    </script>
		

validreg is Onclick Function which is used in input type="button"