This error is generally pointing to the fact that the CIFS routines cannot resolve the name, or connect to the server indicated.
More often than not its because the machine is references by name which is failing to resolve.
Try by using the CIFS/Windows Server IP address.
mount -t cifs -o username=U,password=P //10.65.0.186/c /mnt/windows/
Try also adding
sec=ntlmssp for NTLMv2
mount -t cifs -o username=U,password=P -o sec=ntlmssp //10.65.0.186/c /mnt/windows/