How to run a windows scheduled task 20 mins after system startup?

In windows scheduled tasks properties, you can only choose "at system startup" without being able to assign a specific delay such as 20 minutes, so I wonder how can I setup a schedule task if I want it to run after开发者_运维百科 the system "fully" starts up(you know how fast this can be in XP)? Hope you guys know the answer. Thanks

Create a scheduled task to run an app you've written, have the app you've written sleep for 20 minutes and then run the original app.