Even more on the "unable to fork" error.
It was driving me crazy! I did check that the permissions were set for the IUSR_[server] account to read & exec cmd.exe. Yet still it failed. It wasn't until I got desperate that I happened to look in "special permissions." (Click the advanced tab underneath the regular list of permissions.) There I found that somehow the IUSR_[server] account had a special permission DENYING access! Of course deny overrides everything else.
This was a newly set up install of XP pro. It must be the default of something (I'd guess IIS) to set this deny permission just to make our lives miserable.