Hi David
The answer is, you can't. You need what's called a Cron Job to run your PHP script, which is very easy to set up on a Linux box.
Windows has a task scheduler, but I've never used it. I don't even know whether it's in any way similar to cron jobs. Windows does however have a management console which you reach by entering 'mmc' in the run dialogue and hitting return. You can write a WSH script and then use the MMC to tell the computer to run the script every time it boots or shuts down. Whether you can tell it to run a PHP script is beyond my knowledge. WSH runs JScript and VB-Script. Apparently you can also download modules to deal with Perl and Python.
If you want to use the MMC, I suggest you do some research - it's not exactly intuitive. However, I'm sure there are loads of tips and tricks on the net.
HTH
Norm