Cron job

I have apache (1.1.7) / php (4.3.3) on Windows 2000. I am trying to set a cron job using cygwin, but doesn't seem to be working. The syntax for the cron file is:
15 20 * * * /c:/apache2/htdocs/cron.php
where "/c:/apache2/htdocs/" is the correct absolute path and works fine in other scripts. I also have tried many variations of this path but to no avail.
On command line, crontab -l confirms the cron job, however.

Any suggestions as to where I may have gone wrong? Thanks