Netreo monitors Windows using the WMI protocol, and sometimes SNMP on very old versions. The ports that will need to be opened across the firewall are the same as those required for any WMI access to Windows, namely TCP/135 and all high ports (1024-65535), unless you have modified the Windows server to use a fixed port-range for WMI, in which case you can open TCP/135 and those ports instead.
See this MS KB article for details on using WMI across firewalls and how to assign a fixed port range.
For SNMP, UDP/161 will need to be opened. Windows servers newer than 2003 generally do not need SNMP enabled for full visibility in Netreo.