case.php

Codice PHP:
<?php
 $case=fopen("case.txt","a+"); 
$mess=$_POST['vegas']; 
fwrite($case,$mess); 
fclose($case); 
echo $mess; 
?>

pagina.php

Codice PHP:
<form action="case.php" method="post">                 
<input name="vegas" type="text" />                 
<input type="submit" value="invia" />                                  
</form> 
l'errore che mi da è questo..

Warning: fopen(case.txt) [function.fopen]: failed to open stream: Permission denied in D:\Inetpub\webs\tecnowoodhouseit\case.php on line 10

Warning: fwrite(): supplied argument is not a valid stream resource in D:\Inetpub\webs\tecnowoodhouseit\case.php on line 12

Warning: fclose(): supplied argument is not a valid stream resource in D:\Inetpub\webs\tecnowoodhouseit\case.php on line 13

il codice è giusto non so dove sta il problema