Thanks for the reply! I ran a full system scan with the Avira CD and it found some trojans. But after scanning with SAS the system wouldn't shut down. So I had to restart manually and here we were again. I also did a full system scan with the Avira free version from harddisk. It found one trojan but the problem isn't solved.
Thanks a lot for the link! Unfortunately, I didn't get i right with the rescue CD. I ran it first, rebooted my computer and ran SAS. Found the malware, quarantined it and tried to reboot. Here the trouble started. The computer wouldn't shut down and after 1 hour of waiting I did it manually. At restart I got into the rebooting loop and had to start it with the last known configuration. Any suggestions?? Stefan