Thank you Dan Patterson, I checked the steps described at the other page you linked, but none of these existed anymore.
Matthew Bliss, thank you as well. The Folder at HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Esri still existed, so I deleted it (sorry, I forgot I should send it to you, just remembered as I just deleted it).
Now, as I'm starting the installation it says: "ArcGIS Version 0 detected", installs ArcGIS still in the preset/old Folder (wich shouldn't be any problem) and python is now set to be installed at C:\Python27. So there is no error about the not available hard drive partition anymore, the installation starts. But then it get's interrupted with "Error 1325. webchecker is not a valid short file name." and the installation gets cancelled.
Does anyone has any idea?
About the installation log from running the 10.6 setup: do you know where the log file will be? cant find it, so I don't know the path to it and can't do it