Found my answer. According to the below article, this feature has never worked on WinXP systems!
This issue has been reported on:
Microsoft Windows XP
Microsoft Windows XP SP1 or later
Microsoft Windows 2003
Microsoft Windows 2003 SP1 or later
This issue does not occur on earlier Operating Systems such as Microsoft Windows 2000 Service Pack 4.
Microsoft has changed the security model with recent service packs for Windows. Because of these changes, the task type Run when idle encounters issues.
McAfee has determined that the Run when Idle task feature cannot be made to work with newer Microsoft Service Packs. This feature cannot be fixed. This feature will be removed from future versions of McAfee point products.