exec() issue

I thought I'd sent this, but I can't see it in my list, so here goes again :-) I'm having problems trying to exec() some programs. I have two (similar) classes, one of which downloads and installs the Java Runtime, the other does the same thing for MySQL. The Java one works fine. I can see the MySQL one complete the d/l, exec() unzip.exe to uncompress the file, then I can see it start to run SETUP.EXE in the MySQL distribution. At this point it hangs. If I close the window (which terminates my php script), the MySQL installer will start! Is PHP grabbing some sort of a lock on the window system ? I guess if it is, and the MySQL installer also wants it, there's not a lot I can do... I read around, and I've tried closing my (non-existant, AFAIK) session with session_write_close(); but it seemed to make no difference :-( Anyone conquered this one ? Cheers, Simon