this usually happens if the gpc_magic_quotes setting is enabled in your php.ini, you should contact your host to have them disable this setting.
If they are not willing to disable it, you can try adding the following code on line 1 of your cart page:
<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
foreach($_POST as $k => $v) {
$_POST[$k] = stripslashes($v);
}
}
?>