Sending messages between VB-created apps

[Hack]

Here is an scenario where I need it:

User opens my app (web browser).

User then manages to open it again, possibly by opening a .url link file or somehow doing some form of ShellExecute. I then want the 2nd instance to send the 1st instance a message, telling it to open a new window and passing the URL to open as a string, before closing itself (leaving the user with only 1 instance still and none the wiser).

For this, it seems the easiest thing to do is have your app check to see if it is already running, and it so, then not allow a second instance to be executed.

I didn't think this was possible using PostMessage etc. (thought you could only use the specific constants as messages)

Yes, you are right about that and now that I know the specifics of your request, those APIs I mentioned would not do you any good.