In *nix, you can use cron to kick off your script. It's easy to use. Any reference book/site will tell you how to use it.
In Windows, I've used the event scheduler to kick it off via a .bat script. The .bat script contains only one line, the absolute path to PHP.exe, followed by a space, and then the absolute path to the php script. There may be an better way, bit this is pretty easy.
Biggus