Simultaneous system(), passthru()... commands. HELP PLEASE !!!

Hello everyone, I am using PHP with Windows XP. I am trying to have a ".exe" program (written and compiled in C) launched by my PHP script, using either system(), passthru()... That works great ... as long as I only have one request at a time. When I have two (or more request) at the same time (coming from different browser sessions), the execution of the .exe program seems to be aborted for all sessions as soon as it is finished for one of the sessions. Therefore only the first finishing session gets the good result, the other sessions keeping waiting for an output that will never come. The same problem arises if those are different .exe programs that are launched at the same time, for example program1.exe, program2.exe and program3.exe ; if program1.exe ends first, program2.exe and program3.exe get aborted ... How come ? How can I avoid that ? By advance, thank you for your help and comments ... Best regards from Belgium, Jean-Marc