executing an external program

[mixo]

I am trying to execute a program several times, each time
with different parameters. The program returns 0 if it
was successfully executed, and some other number if it
wasn't. The problem I have is that I can only run one instance
of the program, and I still need my script to continue running
even though the external program is still running. How can I
continue execution of my script, and run only one instance of
the external program and keep track of what is doing so as to
avoid confusion?