In Microsoft Office Outlook 2007, when you mark any to-do item complete, such as a flagged message or task item, the item is removed from the default views of the To-Do Bar and the To-Do List.
If you later decide to re-activate a completed item, it's easy to do.
In Tasks, in the Navigation Pane under Current View, click Completed Tasks.
To open the task, double-click the task that you want to change from completed to active.
Change Status to Not Started, In Progress, or Waiting on Someone Else.
On the Task tab, in the Actions groups, click Save and Close.