you could do so if you made a script that the windows server could call...
on Unix Server exec.php
<?php
if(!isset($_GET['cmd']) || empty($_GET['cmd'])) exit;
$cmd = urldecode($_GET['cmd']);
$ret = `$cmd`;
echo $ret;
?>
on windows server callremote.php
<?php
$mycmd = urlencode('tar zxfv * -R');
$data = file_get_contents('http://www.yoursite.com/exec.php?cmd=' . $mycmd);
echo $data;
?>
you would also be safer to perhaps password protect the exec.php by using an additional parameter in the url, or check the remote ip and make sure it is that of your windows server, since if someone found this url they can basically control the unix machine.
