I wanted to do this too. It's quite easy really. The moo.dll is a very long way to do it and if you only want the uptime it's a waste of CPU cycles.
The way I've done it is goto
http://www.microsoft.com/ntserver/nts/downloads/management/uptime/default.asp
and download uptime.exe. Then put it into your %SYSTEMROOT%/SYSTEM32 folder. One you've done that goto a command prompt and type uptime. That should give you the uptime. All working? Great.
Now, in a PHP script enter the following:
<?php
$result = exec("uptime");
echo("$result");
?>
Now run the file from your webserver and the uptime should be shown.
Neat huh? Good luck.
Duncan.