In this dialog you can choose where you want to search for duplicates:
|•||In local drives|
|•||In network drives|
|•||On mobile data storages|
|•||In the folder "my files"|
|•||In other folders.|
Simply choose the desired folder.
Furthermore your can choose the folders that should be skipped during the search. It is recommended to skip at least the program- and system folders to avoid problems.