MAC address

There's a lot of things in this world I dont understand, one of them being the MAC address, I have a laptop that has been partitioned, it has a wi-fi card in, 99.9% of the time the wifi/internet is NOT CONNECTED, on 1 of the partitions I might connect to 1 of 4 wi-fi networks depending on where I am and what info I need, these connections are manual connect only, I have to choose/click to connect. I have mercedes software installed in a fresh partition that never is or has been connected to the wi-fi/internet as it does not need to be , I have used it a couple of times for wiring diagrams, I just went to use it again and it dont work because the key is wrong and the MAC address is different from before, so question is why has it changed and how can I stop it changing?

Thanks