Te paso al costo un comentario encontrado en el manual de PHP...
For Win2k
If you are using php-4.0.6 and receive an err msg of "unable to fork ..." after running "exec("echo 1")", you can do the followings:
1) Download the CVS version at snaps.php.net
or
2) use "exec("cmd /c echo 1")
This bug was reported a year ago, but is somehow not fixed in
http://gtk.php.net/download.php.