File associations - how to force new copy of program

What controls whether a new copy of the associated program is launched on a
Windows XP system when a file in Windows Explorer is double clicked ?

The normal behavior on Windows 200 and 98 is to start a new copy of the
program.

For example, set up an association of extension .XXX with program
PROG-XXX.EXE.

On Windows XP, double click TEST1.XXX and PROG-XXX starts showing contents
of file TEST1.XXX. Now double click TEST2.XXX. The first instance of
PROG-XXX now shows file TEST2.XXX. On Win2k and 98 it would start a separate
copy of PROG-XXX showing TEST2.XXX. .

But it is possible to start another copy of a program. .TXT is associated
with Notepad. Double clicking two .TXT files starts two copies of Notepad
on all windows versions.

Question: how can I force a new copy of PROG-XXX on Windows XP?

The software in question is a database access product developed using Visual
Studio and the MDI model. Could Win XP be so smart that it assumes I do not
want multiple instances?

Gerard Horgan..