form per mail e virus

<?php
$receiverMail	= "your@email.com";

$name		= ltrim(rtrim(strip_tags(stripslashes($_POST['name']))));
$email		= ltrim(rtrim(strip_tags(stripslashes($_POST['email']))));
$subject	= ltrim(rtrim(strip_tags(stripslashes($_POST['subject']))));
$msg		= ltrim(rtrim(strip_tags($_POST['msg'])));

$ip		= getenv("REMOTE_ADDR");
$msgformat	= "From: $name ($ip)\nEmail: $email\n\n$msg";

// VALIDATION
if(empty($name) || empty($email) || empty($subject) || empty($msg)) {
	echo "<h3>The email was not sent</h3><p>Please fill all the required fields</p>";
}
elseif(!ereg("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)) {
	echo "<h3>The email was not sent</h3><p>The email address is invalid</p>";
}
else {
	mail($receiverMail, $subject, $msgformat, "From: $name <$email>");
	echo "<h3>The email has been sent!</h3><p>I will get back to you as soon as possible.</p>"; }
?>

<form method="post" action="send.php">
Name:<br /><input name="name" type="text" size="30" maxlength="40" /><br /><br />
Email:<br /><input name="email" type="text" size="30" maxlength="40" /><br /><br />
Subject:<br /><input name="subject" type="text" size="30" maxlength="40" /><br /><br />
Message:<br /><textarea name="msg" cols="50" rows="6"></textarea><br /><br />
<input type="reset" value="Reset" />&nbsp;<input type="submit" value="Send" />
</form>

http://www.pcinformatica.eu/2009/05/28/programmazione/php/creare-captcha-in-php/

http://www.trackback.it/articolo/creare-un-semplice-sistema-captcha-in-php/317/