In order to be able to offer advice to you we really need to know a lot more about your computer. Please read this post to see the sort of information required.
However, although I have never heard of a situation where MM starts automatically with Windows, if you are using Win 10, you can check on the Start-Up tab under Task manager and see whether it is listed there. If it is, right-click and choose Disable.
If you're not sure how to access Task manager, perhaps the easiest method is to right-click on the taskbar at the bottom of your screen and choose the Task manager option.
Knowing exactly which code version of the Music Maker EXE you're running might really be the key here, especially if this issue persists after checking the Windows start-up programs, as per Jeff's instructions. If it's any version 25.x, make sure to uninstall it, and replace it with the current one (27.x):
You won't lose any the content and feature items you own. Just make sure to log back into the in-app store system with the correct MAGIX user account again after the installation.
Your issue oddly reminds me of some problem we had two years ago, when generation 25.x was first tested and released. I thought it was mostly happening in our office, with a version that was never released to the public, though. I might be wrong. Never heard about any such problem again in more recent times.