Do try @hazelnut suggestion and if no luck, here are some others.
The slim build is in .exe format, so any problems you have with 7zip shouldn't effect the installation of CC.
But have you tried the portable version of CC, it is in .zip format so (not knowing what exactly your 7zip problem is) I would uninstall 7zip and just use Windows inbuilt zip extractor to unzip it.
Being 'portable' it doesn't need installing. Simply extract it to the folder it will run from and make a shortcut of the .exe file on the desktop, or where-ever you want.
Also, try installing any of the CC builds from Safe Mode.
Disabling antivirus didn't help. Tried installing in safe mode. that didn't work. I cannot uninstall 7-Zip no matter how I try. I'm afraid if I download the portable version 7-zip will screw it up.
That error message is sometimes caused by a problem with Windows Temp folder, which apparently Windows uses no matter where you download programs to or where you install them to. That is, according to some brief research I did.
Go to "Star\Run" and enter "%temp%" (no quotes) and press "OK", and try to delete everything you find in the folder which appears. If some items won't delete it will be because they're in use by another program. Just skip those.
You can't do any harm by deleting stuff in the temp folder, and if you can't delete anything, then it suggests a problem with the "Permissions" flag on the temp folder. Permissions on that folder do apparently get screwed up sometimes and cause problems with program installs.