you said:

I am closing my SampleExe file by close button of Mdi form
How? With an End statement? Do you use Subclassing at all?

Have you simplified the problem by reducing the code to as little as possible to reproduce the problem?

And finally, what is the ActiveX control or perhaps more importantly, are you certain that it is not the problem (is it a VB control and is it using subclassing for example).

Subclassing is one of the quickest and easiest ways to crash a VB app... Very efficient at doing that I have to add

Regards
Paul Lewis