I did a fair bit of looking this morning and according to the forum over at Magix, the only way to fix it is to either restore your system to a time before you installed their software, or do a reinstall of Windows.
To do a system restore, go to Start -> Program Files -> Accessories -> System Tools -> System Restore. You can then go through the wizard and choose a date prior to Magix screwing up your system. If you've turned system restore off. . . well, that would suck in this case.
Keep in mind that everything you've installed and done since the restore point may be lost. Six of one, half dozen of the other so to speak.