ActiveX is Microsoft's marketing term for a set of software technology based on COM. When someone talks about ActiveX, they typically mean ActiveX Controls [L1 ], which are components (that may or may not have a visible representation in a GUI) that can be embedded in an application. To allow this embedding, ActiveX Controls must implement specific COM interfaces. For example, a Visual Basic application may display a GUI containing widgets which are ActiveX Controls. Internet Explorer can download and run an ActiveX component in the browser window (if you gave permission to install it).
Tcl extensions that support ActiveX in some way include ActiveTcl, TclScript, Optcl, TclControl, Noel Frankinet's AxTcl (well, not really his), Joe Mistachkin's TclBridge, ...
The tcom extension provides basic COM access, and does not specifically support the COM interfaces which make ActiveX work.