There's two slightly different results you could specify here:
1) Find all tasks not completed
2) Omit all tasks from the current found set that are completed
I will assume 1). If you wanted 2) we'll need to modify the script a bit.
Enter Find Mode 
Set Field [Tasks::Completed ; "=" ]
Set Error Capture [on]
Perform Find 
Sort Records [Restore ; no dialog ]
This script finds all tasks records where the completed field is empty. If there is data in that field when the task is not completed, the ste field step will need modification. The script would be run after navigating to the tasks layout--either from the OnLayoutEnter trigger or via a button on that layout. A button on other layouts could be used with this script if you added Go to Layout ["Tasks" (Tasks)] as the first line of this script. That button would then take the user to the layout, find the records and sort them all in one mouse click.
Makes sense. I'll give it a shot and let you know. Thanks a bunch!
You are the man! I just love this program! Thank you.