>I have create an application that it has a module which contols permanent a serial port. One of the signals that can received is that the machine which is connected to the serial port is blocked. So I want to interrupt anything that the user doing and give a message.
>Because their main program is DOS based(DOS Prompt not DOS mode) I have to interrupt it too. I have seen a lot of applications that they do something like that. (Printing problems for example).

Christos,

You can try using MessageBox() and adding 2048 to the buttons number. This will produce a system modal messagebox. This may work on win95, it won't on NT because NT doesn't have any system modal capability in the messagebox().