>Can I rely on this control to be pre-installed on my clients' mashines by MS Windows, or must I distribute it?
>

It may be present, but not necessarily the one you're using. You need to distribute the OCX containing the version you want to use and let the installer determine if it's needed, and performance any registration necessary.

>Where/How can I find the control?

Look at the form that contains it to determine what the control is, and if only a ProgID or CLSID is visible, look it uyp in your development machine's registry.