This is the ActiveTcl 8.5 Community Edition installer. Make sure it's the 8.5 version, not 8.6:

Right click the installer to launch it as Administrator. You won't be able to install it for all users if you don't:

Accept the license agreement:

Install for all users, and accept the default path of C:\Tcl:

C:\Tcl\demos doesn't seem to break anything: 😉

Click the Next button, and let the installer do it's thing:

ActiveTcl is now installed. Click the Finish button to exit.