Okay, for those of you who don't understand German, here's the short version of what I've written in my blog entry (see above):
1. Delete the following files:
2. Replace the following files with the original files from wordpress.org:
3. Open "wp-config.php" and check for malicious code and massive empty lines. Clear it all.
4. My theme is "Arthemia Premium". There's a file which should be deleted, too:
5. Replace timthumb.php with the latest version (http://timthumb.googlecode.com/svn/trunk/timthumb.php).
6. Change your MySQL password and update wp-config.php.
7. Change the secret keys in wp-config.php aswell.
8. Clear your browser cache, cookies etc.