I get an 'Unable to fork' error whenever I use any of the program execution functions.

<?php
   exec("c:\inetpub\wwwroot\gp\gnuplot.exe plot.data");
?>

Reports:
'Unable to fork [c:\inetpub\wwwroot\gp\gnuplot.exe plot.data]'

But it is right, right?

Im running IIS 5.1 under WinXP and PHP 4.2.1 in ISAPI mode.

I saw some other bug reports dealing with this problem, but they claim that it was fixed in 4.0.6/7.