Asyncron execution with "start"...

I tried to start an asyncroniously job on a win2003 server with php 4.3.4 (november build) with system("start ...") and passthru("start ...") however it turns out that the php call is waiting until completion of the command that starts invoke.. Thinking about it, it seams like start does fork a bit different then when invoked from a command shell, i.e, start connects php with whatever it starts up, but should it actually behave like this? Nota bene: I've manage to start up async jobs with php, by other means. Any comments?