The %SystemRoot%\Assembly folder is the global assembly cache.
You cannot directly use Windows Explorer to edit ACLs for this folder. Instead, use a command prompt and run the following command:
cacls %windir%\assembly /e /t /p domain\useraccount:r
Alternatively, prior to using Windows Explorer, unregister Shfusion.dll with the following command to give permissions via the GUI:
C:\WINDOWS\Microsoft.NET\Framework\VersionNumber>regsvr32–u shfusion.dll
After setting permissions with Windows Explorer, re-register Shfusion.dll with the following command:
C:\WINDOWS\Microsoft.NET\Framework\VersionNumber>regsvr32 shfusion.dll