I had the following problem:
For any reason my desktop window manager was broken.
This entailed that the task strip and the window strip wasn't transparent anymore, also some other Windows Aero functions were broken. I searched for a long time in the internet, to find a solution, but I didn't find one. But now, I found a solution on myself:
If you are having the same problem, you only have to lie in the Windows7-DVD in the disk drive. Then wait for the autostart of "setup.exe". Then follow the setup until you have the choice to "upgrade" or to "reinstall" Windows7. Here you take "upgrade". This could last until an hour, but you won't have to reinstall all of your programms and desktop-icons. When the setup has finished and you rebooted your computer, the aero effects should work again, but now you're not allowed anymore to run the "Windows-achievement index", because if you will do that, your desktop window manager will be broken again!