Hello!
Now, a few weeks later i found the reason of the report from Avira Internet Security 2013.
After uninstalling Start8 there were some entrys in the registry which were not removed by the Start8 Uninstaller.
Avira IS noticed this and reported this as possible maleware (just if you set "check the integrity of systemfiles" in Aviras configuration). Now i cleaned the registry and removed all Start8 entrys by hand.
Everything works fine now again.
I think that the developer of the uninstaller for Start8 should program this better.
Have a nice week