"Web Install" and "Web Update" are two different animals. I think you are talking about "Web Update" to add live Internet version updating to your program (like Symantech's LiveUpdate).

Web Update consists of two separate components: a client-side executable that runs on the user's system and server-side "cluster" files.

You install a small client application along with your software to give the option to check for newer versions of your applications. The client downloads the latest SetupScript (installation script) from your server to manage your software updates over the Internet.

It's completely up to you what to do with client A and client B. You have full access to the SetupScript scripting language. If client B does not have installed YaddaYadda (your condition goes here) then refuse to update.

Does this help?