Windows XP has a task scheduler built-in that should allow you to do this (Start, All Programs, Accessories, System Tools, Scheduled Tasks).
Also many AV/ anti-spyware apps have their own task scheduling built-in for doing scans, getting updates etc so it may be worth investigating this route first.
*edit* Spybot does have this option if you go into Advanced Mode,Settings and Scheduler , and appears based upon Windows' own scheduled tasks wizard.