I found that COM was giving NULL pointer exceptions for some programs and not others. I tried the following script:

<?
  $quota = new COM("Microsoft.DiskQuota.1");
  $wordpad = new COM("Wordpad.Document.1");
?>

and got:

Fatal error: NULL pointer exception in D:\Inetpub\polytranslive\comtest.php on line 3

I even found that sometimes one instance would work, and another wouldn't, both on the same COM server.