With PHP on Windows, if you get the 'Warning: shell_exec() [function.shell-exec]: Unable to execute' error, then you need to check the permissions on file 'C:\WINDOWS\system32\cmd.exe'. You need read/execute permission on this file. I would recommend using the sysinternals Process Monitor 'procmon.exe' to confirm the user that is trying to run 'cmd.exe'. Filter on 'Process Name' is 'php-cgi.exe' and 'Path' ends with 'cmd.exe'. Look at the event properties for the task with the access denied error, and it will show you the 'Impersonating' user name. This is usually the 'Internet Guest Account', often 'NT AUTHORITY\IUSR'.
