I generally run Linux but I think I recall a post about a similar issue with vBulletin forums when run in ISAPI mode.
Are you perhaps running PHP version 5.1.x? If so, I think you may have run into an ISAPI bug with PHP. I *think* many of those issues have been resolved in PHP 5.2 so having your host upgrade if possible, may be one solution.
Found the reference. See:
May not be related but perhaps that will shed some light. PHP 5.2 has been released since those posts, so that may be one solution to the issue.
Hope this helps.