Please Help: Run Apps from other App.

How to 'Execute' and 'Close' other apps. from one
How to avoid most common mistake of doing this?

Here is my big problem.

I have an app. which uses 4 PC cards with their .dll
linked. No problem to load them all into the app. But after
function of one .dll is called, when certain (not any)
function of other
.dll is called, the app. hands up for good. My debugging
becomes rebooting. I don't know how many times I do a day.

To make sure all pointers are in good standing, I cleared
must of things and made the app. very clean.

But they all work perfectly individually and I tried to
execute them
at once on the screen. No problem.

So, I am thinking to solve the problem by creating
individual apps.
and 'Execute' and 'Close' them from other 'Main' app. Small
needed to pass from each other can be done by writing and
the register file. Large data can be written and read a data

Appreciate if you have other idea to solve the problem.