PHP4 on IIS5 - error using system() or exec()

Hi, I've been trying to execute batch files on my system running PHP 4.0.4 running as ISAPI module on IIS 5 (Win2K). When I execute the commands system() or exec(), I get a "unable to fork" error... I thought that maybe the Guest account for IIS doesn't have the necessary permissions to create new processes, but after putting the IUSR account under the administrators group, there was no difference. Maybe this is a limitation of PHP and it needs to be ran as a CGI-Wrapper to work with system() and exec()? I have tried it on another PC with a similar configuration and it does the same thing, so it's not Windows which needs to be re-installed or something like that... Any suggestions on how to solve that problem would be appreciated.