function isCValid() {
 var searchstring=document.search.searchstring.value;
 var searchfield=document.search.searchfield.value;
 var manufacturer=document.search.manufacturer.value;
 var model=document.search.model.value;

 if (searchstring != "" && searchfield == "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring != "" && manufacturer != "" && model != "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (manufacturer != "" && model != "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring != "" && model != "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring != "" && manufacturer != "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring == "" && manufacturer == "" && model == "") {
	alert('Invalid input combination!')
	return false;
 }
 else {
	return true;
 }
}

function isBValid() {
 var searchstring=document.search.searchstring.value;
 var searchfield=document.search.searchfield.value;

 if (searchstring != "" && searchfield == "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring == "" && searchfield != "") {
	alert('Invalid input combination!')
	return false;
 }
 else if (searchstring == "" && searchfield == "") {
	alert('Invalid input combination!')
	return false;
 }
 else {
	return true;
 }
}