Update: (this is more of a rant...)
I tried Method 1 from this page:
https://support.microsoft.com/de-de/kb/976982
The Adobe Reader has now succesfully uninstalled.
.net-Framework does not install (same error)
Method 2 did not work - error received is 0x80070643
The fix then tried to repair net-framework.
Tool is unable to repair. Thanks for that.
Well, Method 3
Uninstall net-framework....well, guess what.
I tried that already and learned that uninstalling is not possible for Win 7.
Same error.
Trying to install 1.1 and 3.5, nothing happens for a very long time.
Meanwhile I tried this:
Troubleshooting .NET Framework 4 Install failures | VarunGupta
"Check if following registry keys have correct values:
MsiExecCA32 should end with \Windows\syswow64\msiexec.exe
MsiExecCA64 should end with \Windows\system32\msiexec.exe"
They do.
Next up is the Windows automatic troubleshooter. This one tries to repair windows updates as it seems.
Framework 1.1 and 3.5 have succesfully installed, next up is a reboot.
But first we wait for the automatic troubleshooter...then we try to install the winupdates again.
Alright. It solved some Problems, but created a new one.
Now it's telling me 0x80070057. - guess what! Someone saying this is caused by a SATA-Cable!
This I just can't believe, but maybe I will try it out.
Framework installer still gives 0x80070643.