Un install Magix, go to same place from where you had downloaded the trial version.
During download or when you use Magix do not use
Email, messenger, or any other software.
But before you do any thing after you un install Magix Music Maker trial version. Clear browser cookies and history. Turn off computer re start then go to download site and download again. Now comapre the size of file you had earlier downloaded with this new one if there is a difference or not.
Actually Windows 7 is new and I have read here, user are facing a lot of problem therefore, I think, windows xp, vista. If you got no choice then you can try Windows 7.
Who knows may be on your computer that work fine right?
But you got trial version that is very limited. I guess if you take a look at the above Pdf you will have an idea.
Did you clear browser history and cookies?
One thing I did not understand, when we install Trial version of Magix Music Maker we do not got option to install extra content pack. Extra content packs are for full purchased version. And I am sure this the mistake your doing. You got trial version and you are trying to install content pack that is why you got message as per your screen shot.