function validateFields(){
 if (document.getElementById("txtName").value == ""){
 alert("Field 'Full name' is required");    
 }else if (document.getElementById("txtPhone").value == ""){
 alert("Field 'Phone number' is required");  
 }else if (document.getElementById("txtEmail").value == ""){
 alert("Field 'Email address' is required");  
 }else if(document.getElementById("txtComments").value == ""){
 alert("Field 'Comments' is required");  
 }
}