is addressed in a Microsoft Knowledge Based article here;

That is related to a Windows security update 841873 if that has been installed on your computer, and there is a 'hotfix' available.

However, it seems that there may be a workaround. If you set up your scheduled task then click OK instead of Apply, the task may be accepted instead of rejected with the "Access denied" error.

For more information, Google XP Scheduled Tasks "Access Denied" and this will give you links with further guidance on the issue.

I hope that helps.