Andrew,
I'm guessing that this script doesn't need any user input to function? If so, try running the script from command line:
c:\php.exe -q c:\php_scripts\scriptname.php
If the Win32 Scheduler doesn't allow parameters (I axed Scheduler back in Win95 because it was too damned annoying and now can't remember whether it does or not), try placing that command in a *.bat file and executing the batch file with Scheduler. HTH,
Cheers,
Geoff A. Virgo