What distuinguishes a record for a task that is complete from a task that is not?
There are many options and each depends on what you need to accomplish and how you design your database.
A script that is performed when you bring up the report that performs a find only for "complete" task records. This might be done via a script trigger or you might use a navigation button that uses this script to pull up your report. The script can be simple straight forward find or it can be set up to ask the user for additional criteria which it then automatically adds criteria to limit the results to just incomplete tasks.
A portal can be set up that only lists tasks that are incomplete. Either a portal filter expression or a match field that is part of the relationship can be used to limit the records shown in the portal to only incomplete tasks.