Thanks, but as you can see above i've got Windows 7 64bit and therefore the compatibility mode is grayed out.
But i'm happy to announce that i solve the problem just a minute ago.
I tryed some drivers from the Microsoft Update Catalog
To find it, search for "canon s520" and the last listed driver should be the right. To be sure, click on it and control if the architecture is AMD64.
For the lazy ones, the one that is working for me can you find below. ^^
Hope this helps some other too.