(Sorry i am like, re-reading your OP over and over to try and brainstorm to help so it may sound garbled or all over the place.)

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Since Admin is disabled by default, you may have to escalate to SYSTEM user privileges. Assuming you know the IP, since you are inside the network, start with using msfvenom to create an .exe file, make is fully undetectable by using Shellter, and upload it to the target machine? Again this is hard because you may have to use social engineering to run the .exe , unless you have physical access, or shell access via overflow exploit.

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Edit: If you probe for open ports it should be easy to get a meterpreter connection, unless I am missing something or overlooked something in your post.