librenms-librenms/includes/discovery/os/windows.inc.php

<?php

if (!$os) {
    if (strstr($sysObjectId, '1.3.6.1.4.1.311.1.1.3')) {
        $os = 'windows';
    }

    if (preg_match('/Windows/', $sysDescr)) {
        $os = 'windows';
    }
}