Hi
I am trying to mine from a slave PC that only needs the cmd Window running.
That way connected to my Main rig So i got another one popped on the mining without a interface showing on the slave PC.
By slave PC, do you mean that you are mining with the PC's GPU and CPU? If you're mining with a GPU on NiceHash, you can install NiceHash Operating System (NHOS) on the "slave" PC and utilize your main PC using the NiceHash Miner (software) and manage everything from inside the NiceHash website Console Page.
Also, this is really a altcoin question unless you are mining with some physical dedicated bitcoin hardware.