The feature that permits the Registry to remember the last-used key was added to Windows XP, along with the ability to save often-used keys as Favorites. The only way to disable this feature is to block the Registry from saving the last-used key information. To do this, open the Registry and look for this key:
Right-click on the 'LastKey' value in the right pane and select 'Modify'. Erase everything in the 'Value Data' box and leave it blank then click OK. Now, with the 'RegEdit' key in the left pane highlighted, go to 'Edit->Permissions' at the top of the Registry window. Select a User Name then select the 'Deny' checkbox next to 'Full Control' and click OK.
The Registry will now no longer save the last used key, but remember that you will no longer be able to use the Favorites menu. If you prefer not to disable this feature, a fast way to collapse the menu tree is to hold down the left arrow key on the keyboard until only the My Computer entry is visible then tap on the left arrow key once to bring back the five root keys.