The Discover appliance doesn't do OS fingerprinting as part of its analysis, but if I were to attack a problem like this, I'd build a first list of non-Windows candidates using Discover's Activity Groups. In many environments, the SSH servers, NFS clients and NFS servers have a good bet of being non-Windows devices.
You might also work from the other direction, looking for devices using protocols more typically associated with Windows, such as RDP and MSRPC (RPC), to compile a list of "probably Windows" devices.
If you want to do this type of analysis based on a protocol for which the Discover appliance doesn't maintain an Activity Group (e.g. RDP), you can find protocol-device detail under Metrics > Network > (your Wire Network) > L7 Protocols > Details.