You change the width of the various columns used to display file details in the Explorer by dragging the column header borders. The Explorer saves those columns widths for each individual user in the Registry, so that a user always gets the same column widths from session to session.
If you want to restore the default column layout, you must destroy the key in the Registry where Explorer remembers the column widths. Editing the Registry should be done only by experienced users and with extreme care and the Registry should always be exported or backed up before any changes are made.
1. Start the REGEDIT.EXE program.
2. Go to the key HKEY_CURRENT_USER\SOFTWARE\Microsoft\Current Version\Explorer
3. Click the key DirectoryCols entry and press the Delete key. Confirm that you want to delete this key.
4. Exit the REGEDIT program. When you launch the Explorer again the column widths will be restored to their default values.